Some cleanup for handler invalidation

- Cancel reapplyWindowTransformAnim when STATE_HANDLER_INVALIDATED
- Set mActivity and mRecentsView = null in
  invalidateHandlerWithLauncher() instead of posting from
  invalidateHandler()
- Just in case, ignore onActivityInit() if we're already invalidated

Test: Tap repeatedly in nav region in an app, no crash
Fixes: 186983662
Change-Id: I3c005099b46ebb7c9203bcfcceedf48679dd1965