Abort the predictive back animation if another transition occurs.

Another transition request can occur right after starting back
navigation. In that case, the predictive back animation should be
aborted.

Bug: 438031532
Flag: com.android.window.flags.predictive_back_delay_wm_transition
Test: simulate startPredictiveBackAnimation while another transition is
collecting, verify there won't prepare animation.

Change-Id: Ie715c9bcd003e9b79e30f7cc44224d11a03e9ff3