Separate the predictive back transition request from start navigation.

When a finger-down event occurs, the system gets the current back
navigation info from the window manager but does not trigger the
animation immediately.
Instead, the predictive back animation is only triggered when the
gesture movement has crossed the threshold. Therefore, the shell can
pilfer pointers earlier, before the threshold is crossed, and it will
not trigger an extra transition if nothing happen.

Flag: com.android.window.flags.predictive_back_delay_wm_transition
Bug: 301195601
Bug: 418854576
Test: atest BackNavigationControllerTests BackAnimationControllerTest
Change-Id: If6beb4dcfea1c87df870fc23072eb8aeb7db881e