Fix updating a view z-index on Android

Summary:
If the z-index was updated after the initial mount, changes would not be reflected because we did not recalculate the z-index mapped child views and redraw the view. This adds code to do that and call it whenever we update z-index.

**Test plan**
Tested by reproducing the bug with 2 overlapping views that change z-index every second. Made sure it now works properly and z-index changes are reflected.
Closes https://github.com/facebook/react-native/pull/15203

Differential Revision: D5564832

Pulled By: achen1

fbshipit-source-id: 5b6c20147211ce0b7e8954d60f8614eafe128fb4

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

@@ -419,6 +419,13 @@ public class ReactViewGroup extends ViewGroup implements
     }
   }
 
+  @Override
+  public void updateDrawingOrder() {
+    mDrawingOrderHelper.update();
+    setChildrenDrawingOrderEnabled(mDrawingOrderHelper.shouldEnableCustomDrawingOrder());
+    invalidate();
+  }
+
   @Override
   public PointerEvents getPointerEvents() {
     return mPointerEvents;