[flexiglass] Add SceneTransitionLayout (STL) overlays to the framework.

This CL integrates overlays and defines a basic API, based off of
ag/28338922. Overlay transition state isn't included in this initial
version.

Bug: 359173565
Flag: com.android.systemui.scene_container
Test: Added a few unit tests, existing ones still pass. More thorough
unit tests will be added in a follow-up CL, once we've defined a couple
of overlays.

Change-Id: I0fb11d094ce2d7e14e3860b2e0f0f032e320b8d8