Introduce overlays in SceneTransitionLayout (1/2)

This CL adds a first basic support for overlays. Overlays can be either
shown or hidden, and STLState.currentOverlays holds the "effective" set
of current overlays.

There is no overlay transitions in this CL yet, they will be added in
ag/28576084.

See go/sysui-stl-overlay for more details about overlays.

Bug: 353679003
Test: atest OverlayTest
Flag: com.android.systemui.scene_container
Change-Id: Ic0b26a2b7bff41829c409aed3293c47da47db3f5