Highlight the current talkback-focused element in the accessibility tree

Summary: Highlights the element corresponding to the view talkback is focused on in green in the ax tree (and updates live as talkback moves).

Reviewed By: blavalla

Differential Revision: D9021542

fbshipit-source-id: c3bf6f5625aacb0cd054032b33a50541b88b2eaf

src/fb-stubs/AXLayoutExtender.js

@@ -14,6 +14,7 @@ export class AXElementsInspector extends Component<{
   onElementHovered: ?(key: ?ElementID) => void,
   onValueChanged: ?(path: Array<string>, val: any) => void,
   selected: ?ElementID,
+  focused: ?ElementID,
   searchResults?: ?ElementSearchResultSet,
   root: ?ElementID,
   elements: {[key: ElementID]: Element},