Expose onTransition(Start|End) callbacks in STLState

This CL exposes new callbacks on STLState when a transition is
started/finished, as well as a new Transition.cuj coming from the
transition spec.

Bug: 387321113
Test: atest SceneTransitionLayoutStateTest
Flag: com.android.systemui.scene_container
Change-Id: Id0ab0e8a8ac65bdc36afcdae77891bec601eb6f2