Fix issue with stack not invalidating when only focus state changes.

- Adding callback to invalidate when focus state changes.
- Fixing issue where we were posting the invalidate, causing everything
  to lag by 1 frame
- Fixing issue where we weren’t passing the cancel-enter animation flag
  down when setting focused tasks

Change-Id: I218ec78dc0e4e22e59ffcef03a1bc91d08b7c18b