Add ObservableTransitionState.OverlayTransition subclass

This makes ObservableTransition more consistent with TransitionState
which already has TransitionState.OverlayTransition.

Bug: 353679003
Test: atest ObservableTransitionStateTest
Flag: com.android.systemui.scene_container
Change-Id: Id2cedb490df87e01cd2523a080c0400ff0af6e29
1 file changed