Do not jumpcut PiP alpha anim on split-to-pip

Split-to-PiP interactions might start a dismiss split
transition; and if such transition isn't a no-op
(such as when a direct enter PiP comes from either stage),
this follow-up transition might try to get merged into the original
split-to-pip, constantly jumpcutting to PiP instead of cross-fading.

Instead we want to finish running the PiP animation and schedule dismiss
split after.

Since split-to-pip enter CUJs are being fixed, we can also run
FromSplitScreenEnterPipOnUserLeaveHintTest and
FromSplitScreenAutoEnterPipOnGoToHomeTest in PiP2 as well now.

Bug: 388282194
Flag: com.android.wm.shell.enable_pip2
Test: atest FromSplitScreenAutoEnterPipOnGoToHomeTest
Test: atest FromSplitScreenEnterPipOnUserLeaveHintTest
Change-Id: I6c138e38c4af1bbf99a696289b648a119e61b692