[PB] Ensure BAC receive animation leashes before transition.

Fix the regression from ag/28959956.
The onAnimationStart signal is driven by WindowAnimator#animate, so it
could be slower than surface placement because it's following the
frame rate.

Flag: com.android.window.flags.migrate_predictive_back_transition
Bug: 361726487
Test: start activity then trigger back before open transition finish,
verify Shell side will receive animation leashes before transition.

Change-Id: Iea2f4bff7e3bbfb3fc5bb05b46e8b77441e750e7