For cross-activity animation, support seekable animation if app
has customize activity exit transition by Window#setWindowAnimations or
android:windowAnimationStyle.
Because there could fall back to default cross-activity animation when
the customized animation was not able to load, defer assigning the
mActiveCallback when received onAnimationStart.
Bug: 259427810
Test: verify customized animation can play.
Test: atest BackNavigationControllerTests BackAnimationControllerTest\
CustomizeActivityAnimationTest
Change-Id: I59dc6ef75a226c634b06f483aeed7aec03087d18
946871eb75