Fix BackAnimationController race condition

This CL fixes a race condition in BackAnimationController. Since
onThresholdCrossed is not executed on the shell main thread, the
ordering between other BackAnimation API function calls was not
guaranteed. Namely, onThresholdCrossed could have been executed before
the first MOVE event was processed, resulting in  startBackAnimation not
being called. In that case, BackAnimationController injected legacy back
events as a fallback instead of playing the predictive back animation.
Executing onThresholdCrossed on the shell main thread instead should
resolve this problem.

Additionally, the recently implemented small related change (commit 13423234) in EBGH is
put behind the predictiveBackDelayWmTransition flag as well.

Bug: 401832380
Flag: com.android.systemui.predictive_back_delay_wm_transition
Test: BackGestureInvokedTest
Change-Id: I77d0d1de66934754cc64330130d4bd34b4634796