[PiP2] Track display change transitions in PiP

Use a transition observer to track display changing transitions
and update the PiP state if such transition is also changing PiP.

Also make sure PipTransitionState::isPipStateIdle() -> false
while the display change transition involving a PiP change is ongoing.
This should delay any KCA updates or other PiP bounds change transitions
from being scheduled until display change transition is finished and
until the PipBoundsState has been properly updated.

Bug: 382151065
Flag: com.android.wm.shell.enable_pip2
Test: atest PipDisplayChangeObserverTest
Test: atest PipTransitionStateTest
Test: repeatedly fold/unfold while in PiP
Change-Id: Idaf15bd79f18f30e5152bcd3416101dd5522d99c