RN: Add Overflow Visible Kill Switch (Android)

Summary:
Adds a kill switch that reverts the default behavior of `overflow` to be hidden again. The intent of this kill switch is to give applications more time to migrate if necessary (e.g. if they are depending on third party packages with native components that are not compatible with `overflow` being visible by default).
To use the flag, simply set:
import com.facebook.react.views.view.ReactViewGroup;
// Somewhere in the initialization of your application.
ReactViewGroup.sDefaultOverflowHidden = true;
Reviewed By: achen1
Differential Revision: D8718963
fbshipit-source-id: 0eb9aee45dfe04e9ae34d86e3bedcd30a185ef82

Author: yungsters

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

@@ -49,6 +49,11 @@ public class ReactViewGroup extends ViewGroup implements
 ReactInterceptingViewGroup, ReactClippingViewGroup, ReactPointerEventsView, ReactHitSlopView,
 ReactZIndexedViewGroup {
 
+/**
+ * Kill switch to make overflow hidden by default. This flag will eventually be removed.
+ */
+publicstaticbooleansDefaultOverflowHidden;
+
 privatestaticfinalintARRAY_CAPACITY_INCREMENT = 12;
 privatestaticfinalintDEFAULT_BACKGROUND_COLOR = Color.TRANSPARENT;
 privatestaticfinalLayoutParamssDefaultLayoutParam = newViewGroup.LayoutParams(0, 0);
@@ -113,7 +118,10 @@ public void onLayoutChange(
 
 publicReactViewGroup(Contextcontext) {
 super(context);
-setClipChildren(false);
+// TODO: Remove this check after a couple public releases.
+if (!sDefaultOverflowHidden) {
+setClipChildren(false);
+ }
 mDrawingOrderHelper = newViewGroupDrawingOrderHelper(this);
  }