[PB] Only handle one merge transition request.

App can request another transition after the first close transition,
e.g. the second stage transition for continues close activity. Ignore
the following transition to ask other handlers.

Flag: com.android.window.flags.migrate_predictive_back_transition
Bug: 361726487
Test: create a three level+ activities. When finish the top activity,
also finish the second activity to trigger a separate close transition.
Verify the second stage close transition be played with default
transition handler so no jumpcut occur.

Change-Id: I336f2ee302c04b369ca42a804c5ea0ad48eda640