Split the return animation framework flag into two.

- The original one controls core return functionality and ephemeral
  returns
- The new one specifically controls long-lived registrations

This way we can start rolling out the feature to features that don't
require long-lived functionality (e.g. Toast) without having to wait for
it to be complete.

Note that the original flag was never rolled out at all, so it's safe to
do this split now.

Bug: 323863002
Flag: com.android.systemui.shared.return_animation_framework_library
Flag: com.android.systemui.shared.return_animation_framework_long_lived
Test: atest ActivityTransitionAnimatorTest
Change-Id: I67c3fb1843803fd2fe7c65280094fc9714627288