Introduce animation takeovers to ActivityTransitionAnimator.

I want to add motion tests to this class too, but that is a larger
effort better suited for its own CL.

Bug: 323863002
Flag: com.android.systemui.shared.return_animation_framework_library
Flag: com.android.systemui.shared.return_animation_framework_long_lived
Test: atest ActivityTransitionAnimatorTest
Test: manual with flags on and off
Change-Id: Ib545f5bdce953d7fb033334edc3ab7f9d19e8a58