Make AndroidView an interface

Summary:
The current AndroidView stipulates that the backing shadow node can't
be a FlatShadowNode. In some cases, however, we want to apply some of the same
logic (ex not adding NodeRegions, etc) to other ViewManagers that have a
FlatShadowNode backing (and that don't necessarily create a FlatViewGroup).
This commit renames AndroidView to NativeViewWrapper, and re-introduces
AndroidView as an interface, so that logic for padding, NodeRegions, etc can
be shared.

Differential Revision: D2942387

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

@@ -116,7 +116,7 @@ public class FlatUIImplementation extends UIImplementation {
     }
 
     ViewManager viewManager = resolveViewManager(className);
-    return new AndroidView(viewManager);
+    return new NativeViewWrapper(viewManager);
   }
 
   @Override