Defer showing pip menu activity until pip animation ends

This was causes additional churn in the system delaying the visibility
of the activity behind the one we are pipping. We now delay starting the
pip menu activity until the enter pip animation is done.
This doesn't fix the real issue where we don't wait for app transition
to be finished before starting pip, but it does make it much harder to
the user to trigger the case. Will do that in a follow-up CL.

Test: manual
Bug: 37420370
Change-Id: Ic0271d693d45c3e414b3abdf0ad24be44ff5ae5c