PiP: Move enter-PiP on-pause logic.

Previously, we rely on #makeInvisible in other situations as the last
place to enter PIP before activity gets put to stopped (e.g. ARC++).
However with Shell transitions, #makeInvisible is now immediately called
as user starts the transition into Recents, which would cause app to
enter PIP right away. Thus, removing this part of the code, and instead
we will wait until the Recents transition is finished and try to pause
the app with userLeaveHint before visibility is committed.

Also, when shell transition is enabled, make an explicit transition when
dmissing PIP to properly commit the necessary window container changes
to WM. The tranition will just have no animation.

Bug: 165793917
Test: Enable Shell transition, enter PIP via gesture (non-auto enter
case)
Test: Enable Shell transition, dismiss PIP via dismiss bubble

Change-Id: Ic2c8691f5a1f359b822b1874b16a327bc689f34d