Make fragment leftover from an attach/detach race slightly safer (flutter#29908)

Author: ksballetba

shell/platform/android/io/flutter/embedding/android/FlutterFragment.java

@@ -784,7 +784,9 @@ public void onResume() {
   // possible.
   @ActivityCallThrough
   public void onPostResume() {
-    delegate.onPostResume();
+    if (stillAttachedForEvent("onPostResume")) {
+      delegate.onPostResume();
+    }
   }
 
   @Override

shell/platform/android/test/io/flutter/embedding/android/FlutterFragmentTest.java

@@ -121,9 +121,11 @@ public void itCanBeDetachedFromTheEngineAndStopSendingFurtherEvents() {
     fragment.setDelegate(mockDelegate);
     fragment.onStart();
     fragment.onResume();
+    fragment.onPostResume();
 
     verify(mockDelegate, times(1)).onStart();
     verify(mockDelegate, times(1)).onResume();
+    verify(mockDelegate, times(1)).onPostResume();
 
     fragment.onPause();
     fragment.detachFromFlutterEngine();
@@ -136,9 +138,11 @@ public void itCanBeDetachedFromTheEngineAndStopSendingFurtherEvents() {
 
     fragment.onStart();
     fragment.onResume();
+    fragment.onPostResume();
     // No more events through to the delegate.
     verify(mockDelegate, times(1)).onStart();
     verify(mockDelegate, times(1)).onResume();
+    verify(mockDelegate, times(1)).onPostResume();
 
     fragment.onDestroy();
     // 1 time same as before.