Allow a back gesture to interrupt a transition.

This change allows a new back gesture to start without waiting for the
previous transition to finish. If the back gesture has not finished
after the previous transition completes, attempt to start the back
navigation again.

Flag: com.android.window.flags.predictive_back_intercept_transition
Bug: 426448018
Test: atest BackAnimationControllerTest BackNavigationControllerTests
Change-Id: Ice87d6d7353d537d1943d92125ef82c8e8d8d9ff