Fix Viewpager on Android when using native navigation.

Summary:
See the "broken" video attached to really understand the problem easily.

On Android after navigating to any other screen using wix navigation library, the native viewpager would lose the settling page behaviour which is quite annoying for the users.

This is caused by the onAttachedToWindow that resets mFirstLayout to true inside ViewPager. By request another layout pass, everything works as expected.

Working video is the application with patched RN.

[broken.mp4](https://github.com/facebook/react-native/files/1128028/broken.mp4.zip)
[working.mp4](https://github.com/facebook/react-native/files/1128032/working.mp4.zip)
Closes facebook/react-native#14867

Differential Revision: D7154981

Pulled By: hramos

fbshipit-source-id: 2b3570800a5320ed2c12c488748d9e1358936c84

ReactViewPager.java

@@ -210,6 +210,26 @@ public void setScrollEnabled(boolean scrollEnabled) {
     mScrollEnabled = scrollEnabled;
   }
 
+
+  @Override
+  protected void onAttachedToWindow() {
+    super.onAttachedToWindow();
+    // The viewpager reset an internal flag on this method so we need to run another layout pass
+    // after attaching to window.
+    this.requestLayout();
+    post(measureAndLayout);
+  }
+
+  private final Runnable measureAndLayout = new Runnable() {
+    @Override
+    public void run() {
+      measure(
+              MeasureSpec.makeMeasureSpec(getWidth(), MeasureSpec.EXACTLY),
+              MeasureSpec.makeMeasureSpec(getHeight(), MeasureSpec.EXACTLY));
+      layout(getLeft(), getTop(), getRight(), getBottom());
+    }
+  };
+
   /*package*/ void addViewToAdapter(View child, int index) {
     getAdapter().addView(child, index);
   }