Completely remove pinned task when starting a new PiP

When a new ActivityRecord tries to replace the existing PiP by
explicitly call enterPictureInPictureMode (normally a cause of in-app
enter PiP button), it's more desirable that the old PiP disappears
instead of turning into full-screen at the same time, as the
Task#dismissPip is trying to do.

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/ebskmHfvlcwKa3OSBFONWY
Bug: 231294285
Test: following the reproduce steps, see video
Test: atest PinnedStackTests
Change-Id: Ib4fb9be03d1a26a7046c303124224b0ecccabecb