Refactor PipDisplayChangeObserver to avoid flicker

This CL does a couple things as we are refactoring
PipDisplayChangeObserver:
 - Restrict isDisplayChangeScheduled flag to cover the time
between display change transition being requested in PipController
and the point when this transition starts playing. This will keep
PiP transition state as non-idle

 - Reuse normal PiP bounds change PipTransitionState states to block
input events just like for other PiP bounds change transitions
but while only while display change with PiP change is playing.

 - Also starting to cache display change transitions in a map since
they can come in multiples one after another, and we don't want to
override the token cached at ready (could be queued)
before the transition starts playing.

Bug: 431703892
Flag: EXEMPT bugfix
Test: atest WMShellUnitTests:PipTransitionStateTest
Test: atest WMShellUnitTests:PipDisplayChangeObserverTest
Change-Id: Id99fdc4c92acc1db0081c57d3396751e183b52fb