Create close_prepare_back transition after animation finish.

User can interrupt cancel post commit animation and reuse the existing
animation leashes, so better to create the close-prepare-back
transition after previous cancel animation finished, to preventing
from create more transition during single gesture animation.

Flag: com.android.window.flags.migrate_predictive_back_transition
Bug: 365755045
Test: atest BackAnimationControllerTest
Test: trigger back-to-home gesture and interrupt the cancel animation,
verify there is only one close_prepare_back transition created after
animation finished.

Change-Id: Ib209fcafe2098a932b7524920b235d236660b98e