Add 'Share' button to the selection toolbar on Android (flutter#139479)

## Description

This PR adds the 'Share' button to the text selection toolbar on Android.

## Related Issue

Fixes flutter#138728

## Tests

Refactor a lot of existing tests in order to:
- make them more readable (avoid duplication by introducing helper functions, specify explictly check which buttons are expected).
- make them more accurate (check that expected buttons are visible instead of just checking the number of buttons).

For instance, previous tests contained sections such as:

```dart

      // Collapsed toolbar shows 3 buttons.
      expect(
        find.byType(CupertinoButton),
        isContextMenuProvidedByPlatform ? findsNothing : isTargetPlatformIOS ? findsNWidgets(6) : findsNWidgets(3)
      );

```

Where the comment is obsolete, the two cases (6 widgets and 3 widgets) are not explicit (which buttons are expected?), and not accurate (will pass if the number of buttons is right but the buttons are the wrong ones).

packages/flutter/lib/src/material/adaptive_text_selection_toolbar.dart

@@ -226,7 +226,7 @@ class AdaptiveTextSelectionToolbar extends StatelessWidget {
           case ContextMenuButtonType.searchWeb:
             return localizations.searchWebButtonLabel;
           case ContextMenuButtonType.share:
-            return localizations.searchWebButtonLabel;
+            return localizations.shareButtonLabel;
           case ContextMenuButtonType.liveTextInput:
             return localizations.scanTextButtonLabel;
           case ContextMenuButtonType.custom:

packages/flutter/lib/src/widgets/editable_text.dart

@@ -1882,6 +1882,10 @@ class EditableText extends StatefulWidget {
       // If the paste button is enabled, don't render anything until the state
       // of the clipboard is known, since it's used to determine if paste is
       // shown.
+
+      // On Android, the share button is before the select all button.
+      final bool showShareBeforeSelectAll = defaultTargetPlatform == TargetPlatform.android;
+
       resultButtonItem.addAll(<ContextMenuButtonItem>[
         if (onCut != null)
           ContextMenuButtonItem(
@@ -1898,6 +1902,11 @@ class EditableText extends StatefulWidget {
             onPressed: onPaste,
             type: ContextMenuButtonType.paste,
           ),
+        if (onShare != null && showShareBeforeSelectAll)
+          ContextMenuButtonItem(
+            onPressed: onShare,
+            type: ContextMenuButtonType.share,
+          ),
         if (onSelectAll != null)
           ContextMenuButtonItem(
             onPressed: onSelectAll,
@@ -1913,7 +1922,7 @@ class EditableText extends StatefulWidget {
             onPressed: onSearchWeb,
             type: ContextMenuButtonType.searchWeb,
           ),
-        if (onShare != null)
+        if (onShare != null && !showShareBeforeSelectAll)
           ContextMenuButtonItem(
             onPressed: onShare,
             type: ContextMenuButtonType.share,
@@ -2300,13 +2309,18 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
 
   @override
   bool get shareEnabled {
-    if (defaultTargetPlatform != TargetPlatform.iOS) {
-      return false;
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.android:
+      case TargetPlatform.iOS:
+        return !widget.obscureText
+            && !textEditingValue.selection.isCollapsed
+            && textEditingValue.selection.textInside(textEditingValue.text).trim() != '';
+      case TargetPlatform.macOS:
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.linux:
+      case TargetPlatform.windows:
+        return false;
     }
-
-    return !widget.obscureText
-        && !textEditingValue.selection.isCollapsed
-        && textEditingValue.selection.textInside(textEditingValue.text).trim() != '';
   }
 
   @override
@@ -2516,9 +2530,9 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
   }
 
   /// Launch the share interface for the current selection,
-  /// as in the "Share" edit menu button on iOS.
+  /// as in the "Share..." edit menu button on iOS.
   ///
-  /// Currently this is only implemented for iOS.
+  /// Currently this is only implemented for iOS and Android.
   ///
   /// When 'obscureText' is true or the selection is empty,
   /// this function will not do anything

packages/flutter/test/cupertino/adaptive_text_selection_toolbar_test.dart

@@ -10,6 +10,7 @@ import 'package:leak_tracker_flutter_testing/leak_tracker_flutter_testing.dart';
 
 import '../widgets/clipboard_utils.dart';
 import '../widgets/live_text_utils.dart';
+import '../widgets/text_selection_toolbar_utils.dart';
 
 void main() {
   final MockClipboard mockClipboard = MockClipboard();
@@ -31,13 +32,6 @@ void main() {
     );
   });
 
-  Finder findOverflowNextButton() {
-    return find.byWidgetPredicate((Widget widget) =>
-      widget is CustomPaint &&
-          '${widget.painter?.runtimeType}' == '_RightCupertinoChevronPainter',
-      );
-  }
-
   testWidgetsWithLeakTracking('Builds the right toolbar on each platform, including web, and shows buttonItems', (WidgetTester tester) async {
     const String buttonText = 'Click me';
 
@@ -188,27 +182,55 @@ void main() {
     ));
 
     expect(find.byKey(key), findsOneWidget);
-    expect(find.text('Copy'), findsOneWidget);
-    expect(find.text('Cut'), findsOneWidget);
-    expect(find.text('Select All'), findsOneWidget);
-    expect(find.text('Paste'), findsOneWidget);
-    expect(find.text('Look Up'), findsOneWidget);
 
     switch (defaultTargetPlatform) {
       case TargetPlatform.android:
         expect(find.byType(CupertinoTextSelectionToolbarButton), findsNWidgets(6));
+        expect(find.text('Cut'), findsOneWidget);
+        expect(find.text('Copy'), findsOneWidget);
+        expect(find.text('Paste'), findsOneWidget);
+        expect(find.text('Select All'), findsOneWidget);
+        expect(find.text('Share...'), findsOneWidget);
+        expect(findCupertinoOverflowNextButton(), findsOneWidget);
+
+        await tapCupertinoOverflowNextButton(tester);
+
+        expect(find.byType(CupertinoTextSelectionToolbarButton), findsNWidgets(4));
+        expect(findCupertinoOverflowBackButton(), findsOneWidget);
+        expect(find.text('Look Up'), findsOneWidget);
+        expect(find.text('Search Web'), findsOneWidget);
+        expect(findLiveTextButton(), findsOneWidget);
+
       case TargetPlatform.fuchsia:
-        expect(find.byType(CupertinoTextSelectionToolbarButton), findsNWidgets(6));
       case TargetPlatform.iOS:
         expect(find.byType(CupertinoTextSelectionToolbarButton), findsNWidgets(6));
-        expect(findOverflowNextButton(), findsOneWidget);
-        await tester.tapAt(tester.getCenter(findOverflowNextButton()));
-        await tester.pumpAndSettle();
+        expect(find.text('Cut'), findsOneWidget);
+        expect(find.text('Copy'), findsOneWidget);
+        expect(find.text('Paste'), findsOneWidget);
+        expect(find.text('Select All'), findsOneWidget);
+        expect(find.text('Look Up'), findsOneWidget);
+        expect(findCupertinoOverflowNextButton(), findsOneWidget);
+
+        await tapCupertinoOverflowNextButton(tester);
+
+        expect(find.byType(CupertinoTextSelectionToolbarButton), findsNWidgets(4));
+        expect(findCupertinoOverflowBackButton(), findsOneWidget);
+        expect(find.text('Search Web'), findsOneWidget);
+        expect(find.text('Share...'), findsOneWidget);
         expect(findLiveTextButton(), findsOneWidget);
+
       case TargetPlatform.macOS:
       case TargetPlatform.linux:
       case TargetPlatform.windows:
         expect(find.byType(CupertinoDesktopTextSelectionToolbarButton), findsNWidgets(8));
+        expect(find.text('Cut'), findsOneWidget);
+        expect(find.text('Copy'), findsOneWidget);
+        expect(find.text('Paste'), findsOneWidget);
+        expect(find.text('Select All'), findsOneWidget);
+        expect(find.text('Share...'), findsOneWidget);
+        expect(find.text('Look Up'), findsOneWidget);
+        expect(find.text('Search Web'), findsOneWidget);
+        expect(findLiveTextButton(), findsOneWidget);
     }
   },
     skip: kIsWeb, // [intended] on web the browser handles the context menu.