Allow intermittent EXITING in PiP states

There are certain special cases where we should
allow advancement to CHANGING_PIP_BOUNDS state
while not being in SCHEDULED_BOUNDS_CHANGE state.

This is because activity restart, for instance, can
trigger advancement to EXITING_PIP at the time when
exit is just scheduled (transition isn't playing yet);
this has been used to send all components a signal to finish
any animations running outside a transition (e.g. physics animator).

However, we can totally animate a PiP bounds change transition
scheduled earlier since sequentiality is guaranteed with transitions.

What this CL allows is the following:
SCHEDULED_BOUNDS_CHANGE -> EXITING_PIP -> CHANGING_PIP_BOUNDS

where the latter triggers resize animation in the relevant component.

Finally, we also setState(EXITING_PIP) when starting to play
exit-via-expand animation to avoid a an error log that we are trying to
go from _non_EXITING_PIP -> EXITED_PIP.

This should be fine to try twice since, the second time will be blocked
if redundant by PipTransitionState.

Bug: 434087237
Flag: EXEMPT bugfix
Test: atest PipTransitionState
Test: atest ActivityEmbeddingCrossUidTests
  #testUntrustedCrossUidActivityEmbeddingRestoreFromPip_succeedWhenAppOptIn
Change-Id: I3ef11f1a30f35268c8d888fc7cd7a18d0b96b8fb