Move onChangeScene and transitions to STLState (1/2)

This CL is a refactoring that moves onChangeScene and transitions
to SceneTransitionLayoutState. It also removes the
SceneTransitionLayoutState() factory to create a STLState, and replaces
it with the updateSceneTransitionLayoutState() Composable to create a
STLState from a hoisted source of truth. In ag/25812691, I'm adding a
MutableSceneTransitionLayoutState that can replace current usages of the
SceneTransitionLayoutState() factory outside composition (currently used
only in tests. This means that the tests don't compile with only this
CL and this CL will be submitted together with ag/25812691.

Note that the name updateSceneTransitionLayoutState() was chosen to be
consistent with Compose updateTransition().

Test: PlatformComposeSceneTransitionLayoutTests
Bug: 318794193
Flag: N/A
Change-Id: Ie7519ec4a3c92afb31f6c5fc58074e9d2131d2f5