Add MutableSTLState.snapToScene()

This CL adds a new snapToScene() function to MutableSTLState that can be
used to immediately snap to a scene. This can for instance be useful to
make sure that the Shade is immediately hidden after a launch
transition.

Bug: 330672236
Test: atest SceneTransitionLayoutStateTest
Flag: N/A
Change-Id: I37d59c20d8634bdb9d0486e8726006b6fe29b9cb