Explicitly call out Home vis change on exiting PiP

Explicitly call HomeTransitionObserver#notifyHomeVisibilityChanged when
exiting PiP to full-screen mode.

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/bDCe5aL9ctHwQWlW9lMvAr
Bug: 340443311
Test: Enter and exit PiP in button navigation mode, tap Recents button
Change-Id: I47c948af54db22305f56e0c95df049e2f87abc3c
(cherry picked from commit 0634ed0d)