Defer play PB animation until transition ready.

Only needed when start prepare_open(launchBehind) transition.
When opening target participant transiton, it's surface only become
visible after transition ready, so shell side shouldn't start animation
before transition ready.

Flag: com.android.window.flags.migrate_predictive_back_transition
Bug: 361726487
Test: trigger cross-activity animation without snapshot, verify
animation start with the surface of open activity become visible.

Change-Id: Icc51abe139bfc7164f0842c8eb05e465bd262cdc