PiP: Show the wallpaper as part of the transition.

This is usually taken care by RemoteAnimation, but since PiP takes
control of the entire thing that is no longer the case. So let's just
handle it.

Bug: 165793917
Test: Enter PiP with ENABLE_SHELL_TRANSITIONS flag on
Change-Id: I8f0af4472c20016f856f33f903bd9a431cd5ef3a