Ignore moveTaskToBack transition for PiP

YT calls Activity#moveTaskToBack(false) when tapping on background play
in PiP mode. Simply ignore any system transition in this case.

See also go/pip-move-task-to-back

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/g1i3IAyUeSJua1dA61OhXQ
Bug: 175279237
Test: Tap background play when in PiP, see Video
Change-Id: I7319fe6e12208e51e516bc4ea82ee7325ccecc11