Defer showing taskbar when playing back-to-home animation.

After migrate predictive back to shell transition, the home activity
will participant in the prepare transition instead of after gesture
commit, so HomeTransitionObserver should defer showing taskbar if the
transition change is back gesture animated, and let
LauncherBackAnimationController to control when to show taskbar.

Flag: com.android.window.flags.migrate_predictive_back_transition
Bug: 363927522
Test: atest HomeTransitionObserverTest
Change-Id: Icbfffac12e599fef7a2acce5ab013dd5fdd90831