Fix #6228: Crash in Android dialog module.

Summary:
Android dialog module has a race condition as a result of which it crashes.
See this issue:
#6228.

The mIsInForeground flag is set on UI thread but was used from the ReactMethod thread.
Now all public methods of FragmentManagerHelper called from UI thread only.
Asserts are added in appropriate places to prevent future regressions.

Make sure that dialogs work after this change.
It will be nearly impossible to reproduce the issue manually but automatic regression tests should be able to catch this. At least our tests were crashing on some dialog scenarios from time to time.

[ANDROID] [MINOR] [BUGFIX] - Race condition fix in Android Dialogs module.
Closes #17392

Reviewed By: achen1

Differential Revision: D6708787

Pulled By: mdvacca

fbshipit-source-id: 99beb3ea3046286cc973f7677e98ff36f162b09b

Author: dryganets

ReactAndroid/src/main/java/com/facebook/react/modules/dialog/DialogModule.java

@@ -28,6 +28,7 @@
 import com.facebook.react.bridge.ReactMethod;
 import com.facebook.react.bridge.ReadableArray;
 import com.facebook.react.bridge.ReadableMap;
+import com.facebook.react.bridge.UiThreadUtil;
 import com.facebook.react.common.MapBuilder;
 import com.facebook.react.module.annotations.ReactModule;
 
@@ -95,6 +96,7 @@ public FragmentManagerHelper(android.app.FragmentManager fragmentManager) {
     }
 
     public void showPendingAlert() {
+      UiThreadUtil.assertOnUiThread();
       if (mFragmentToShow == null) {
         return;
       }
@@ -123,6 +125,8 @@ private void dismissExisting() {
     }
 
     public void showNewAlert(boolean isInForeground, Bundle arguments, Callback actionCallback) {
+      UiThreadUtil.assertOnUiThread();
+
       dismissExisting();
 
       AlertFragmentListener actionListener =
@@ -218,8 +222,8 @@ public void onHostResume() {
   public void showAlert(
       ReadableMap options,
       Callback errorCallback,
-      Callback actionCallback) {
-    FragmentManagerHelper fragmentManagerHelper = getFragmentManagerHelper();
+      final Callback actionCallback) {
+    final FragmentManagerHelper fragmentManagerHelper = getFragmentManagerHelper();
     if (fragmentManagerHelper == null) {
       errorCallback.invoke("Tried to show an alert while not attached to an Activity");
       return;
@@ -253,7 +257,13 @@ public void showAlert(
       args.putBoolean(KEY_CANCELABLE, options.getBoolean(KEY_CANCELABLE));
     }
 
-    fragmentManagerHelper.showNewAlert(mIsInForeground, args, actionCallback);
+    UiThreadUtil.runOnUiThread(new Runnable() {
+      @Override
+      public void run() {
+        fragmentManagerHelper.showNewAlert(mIsInForeground, args, actionCallback);
+      }
+    });
+
   }
 
   /**