Turn FlatViewManager into ViewGroupManager

Reviewed By: ahmedre

Differential Revision: D2768667

fb-gh-sync-id: 39298e4033aab5ca8230516f98da252790811f14

ReactAndroid/src/main/java/com/facebook/react/uimanager/ViewGroupManager.java

@@ -24,7 +24,7 @@ public abstract class ViewGroupManager <T extends ViewGroup>
   }
 
   @Override
-  public Class<LayoutShadowNode> getShadowNodeClass() {
+  public Class<? extends LayoutShadowNode> getShadowNodeClass() {
     return LayoutShadowNode.class;
   }