Add STLState.isTransitioning(from?, to?)

This CL adds a SceneTransitionLayoutState.isTransitioning() utility to
easily check whether we are transitiong (optionally between two scenes).
We also pass the STLState in SceneScope so that scenes can easily access
it.

Bug: 291071158
Test: SceneTransitionLayoutStateTest
Flag: NA
Change-Id: Ic5def4bd28222004ad172f24648b589d1819cca4