[PB] Unify predictive back transition.

Always trigger predictive_back transition even if when the animation
is shown as snapshot. So shell side can receive the transition happen
when animation start instead of wait for close transition coming.

Flag: com.android.window.flags.unify_back_navigation_transition
Bug: 372230928
Test: atest BackAnimationControllerTest
Test: verify predictive_back transition should always happen when back
gesture start with animatable situation.

Change-Id: I0a8367de12dc6bb385c8beacc6ecb46e3d0ca1ff