Introduce MutableSceneTransitionLayoutState

This CL introduces MutableSceneTransitionLayoutState, a super class of
SceneTransitionLayoutState that holds the current scene/state. Having
the source of truth in a MutableSceneTransitionLayoutState (rather than
hoisted somewhere else) will allow to provide more APIs for
transitioning to some state.

Bug: 318794193
Test: SceneTransitionLayoutStateTest
Flag: N/A
Change-Id: Ifbad7b46169b77d7f5b3d0cd9b9ad8bc7d281257