Reset NodeRegion when a node gets mounted to a View

Summary: Small optimization. Once we mount a Node to a View, its NodeRegion is not used anymore with RCTText being an exception, but it will correct itself automatically upon updateNodeRegion call. This patch allows the dead object to get garbage collected.

Reviewed By: ahmedre

Differential Revision: D2816331

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

@@ -263,6 +263,9 @@ import com.facebook.react.uimanager.ViewProps;
       if (getParent() != null) {
         invalidate();
       }
+
+      // reset NodeRegion to allow it getting garbage-collected
+      mNodeRegion = NodeRegion.EMPTY;
     }
   }