Add IPipAnimationListener#onExpandPip

This signals Launcher when user leaves PiP mode by tapping on the
expand button in PiP window. Launcher can use signal, for instance, to
hide the Taskbar to avoid flicker when the app settles in full-screen mode.

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/dIgvinb9yEB8MYfYDx0Ijy
Bug: 218450853
Test: see video
Change-Id: I0e9882324daa6dc419ad2746312a22a0c8721ff4