Clear close prepare transition token by TransitionObserver

Transition handler won't receive startAnimation nor mergeAnimation if
there is no changes in that transition. Use TransitionObserver to
track the state in case the close prepare transition is aborted.

Flag: EXEMPT bugfix
Bug: 373828944
Test: disable snapshot, click app button on edge which also trigger
predictive back animation, verify the close prepare transition can
be reset if aborted.

Change-Id: I6c927f5a7602ed3345da05564671445496d30b5a