Fix several bugs in back animation handoff.

1. Always start animation after preparation.
2. Reset handoff handler to null when handoff is not available.

Flag: com.android.systemui.shared.return_animation_framework_library
Flag: com.android.systemui.shared.return_animation_framework_long_lived
Bug: 382137941
Test: manual test with test app
Change-Id: I81e98b8a49c7851e5b14e4983e88b43c2f7a59d0