[2/2][PiP2] Block duplicate exit PiP calls

1. Move the setting of EXITING_PIP state up to where
it's scheduled.

2. Block exit/remove PiP calls if not in PiP
(e.g. if exit is already scheduled)

3. Block the scheduling of resizes if not in PiP
through shouldTransitionToState() checks.

4. Separate the scheduling of a remove transition from
an exit transition - also allow the choice of animation w/o fadeout.

Bug: 381017000
Flag: com.android.wm.shell.enable_pip2
Test: atest PipSchedulerTest
Test: atest PinnedStackTests#testPreventSetAspectRatioWhileExpanding
Test: atest PipTransitionStateTest
Test: manually repro steps in the bug
Change-Id: Ief665f45e85be567c3896c336833a8f48a0894a8