Properly invalidate FlatViewGroup when hotspot Drawable invalidates

Summary: I lost FlatViewGroup.verifyDrawable(Drawable) method during one of the rebases. Re-adding it now because hotspot drawable is not working without it.

Reviewed By: ahmedre

Differential Revision: D2772460

ReactAndroid/src/main/java/com/facebook/react/flat/FlatViewGroup.java

@@ -122,6 +122,11 @@ import com.facebook.react.uimanager.ReactCompoundView;
     // nothing to do here
   }
 
+  @Override
+  protected boolean verifyDrawable(Drawable who) {
+    return who == mHotspot || super.verifyDrawable(who);
+  }
+
   @Override
   protected void onAttachedToWindow() {
     if (mIsAttached) {