Detach the nav bar from the app when going home

- Nothing currently detaches the nav bar from the app until the recents
  animation completes if onAnimationStart() happens after the end
  target is set

Flag: EXEMPT bugfix
Fixes: 343457097
Test: Artificially introduce delayed onRecentsAnimationStart and verify
      detachNavFromApp is called
Change-Id: I1ec0d8821d61f9fad9a811d6a12ce21e8417252f