Expose a getter for overflow setting in ReactViewGroup (#21398)

Summary:
This change adds getter for overflow attribute in ReactViewGroup class. Overflow setting can affect how view children are drawn but also how hit testing behaves when receiving touch. Exposing this setting makes it possible for gesture-handler library to implement proper hit testing that takes into account overflow property of a view.
Pull Request resolved: https://github.com/facebook/react-native/pull/21398

Differential Revision: D10105398

Pulled By: shergin

fbshipit-source-id: 15ae2b31be3bf80e7e1d28b87ce4474af0f486f5

ReactAndroid/src/main/java/com/facebook/react/views/view/ReactViewGroup.java

@@ -644,6 +644,10 @@ public class ReactViewGroup extends ViewGroup implements
     invalidate();
   }
 
+  public @Nullable String getOverflow() {
+    return mOverflow;
+  }
+
   /**
    * Set the background for the view or remove the background. It calls {@link
    * #setBackground(Drawable)} or {@link #setBackgroundDrawable(Drawable)} based on the sdk version.