Remove 'box-only' as a layoutOnlyProp

Summary:
Currently, having the property `pointerEvents={'box-only'}` marks a view as layoutOnly. For optimization reasons, layoutOnly nodes are [not added to the node hierarchy](b103903ec8/ReactAndroid/src/main/java/com/facebook/react/uimanager/NativeViewHierarchyOptimizer.java (L99)) The problem is that should this box ever need to be [transitioned to not-layout-only](b103903ec8/ReactAndroid/src/main/java/com/facebook/react/uimanager/NativeViewHierarchyOptimizer.java (L394)) (for example, because you add the a CSS transform property),  it must be added to the hierarchy. To add it to the hierarchy the [React Styles Diff Map](b103903ec8/ReactAndroid/src/main/java/com/facebook/react/uimanager/NativeViewHierarchyOptimizer.java (L396)) is passed along with the backing shadowNode to create the node. The problem is that at no point were the original `pointerEvents` and so the new node will be created with `pointerEvents =  'auto'`.

A more correct fix to this problem might be to have shadowNodes carry around their pointerEvents, although this is likely a greater design decision which is why am I proposing the quick fix now.

Will also resolve: https://github.com/react-native-community/react-native-modal/issues/11
Closes https://github.com/facebook/react-native/pull/15529

Reviewed By: AaaChiuuu

Differential Revision: D5792012

Pulled By: tomasreimers

fbshipit-source-id: 625242c53e16cb293c64811a57f6c3905b3483e0

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

@@ -169,7 +169,7 @@ public class ViewProps {
       return true;
     } else if (POINTER_EVENTS.equals(prop)) {
       String value = map.getString(prop);
-      return "auto".equals(value) || "box-none".equals(value);
+      return "auto".equals(value);
     } else {
       return false;
     }