Add return animation functionality to ActivityTransitionAnimator.

I spent some time trying to add tests for this, but in the current state
it's really difficult without adding a disproportionate amount of
refactoring. It is also worth noting that the motion testing library
doesn't support RemoteAnimationAdapter, so we're not able to use that to
test window animations.

Bug: 323863002
Flag: ACONFIG com.android.systemui.shared.return_animation_framework_library DISABLED
Test: manually tested existed use cases with the flag on and off (same
behavior)

Change-Id: I8c1c0f0ca1c026eb7165496b6c4ee333c59ce365