Small updates to Layout Extension code (committing to keep separate from larger changes)

Summary: Changed AXVisible checking, added AX specific hover

Reviewed By: danielbuechele

Differential Revision: D8795383

fbshipit-source-id: bb10b9e860629cfd385d97e79587c76460b516bc

src/fb-stubs/AXLayoutExtender.js

@@ -23,3 +23,5 @@ export class AXElementsInspector extends Component<{
     return null;
   }
 }
+
+export const AXToggleButtonEnabled: boolean = false;