[flexiglass] Expose API to instantly show/hide multiple overlays at once

This also consolidates snapToScene, instantly{Show,Hide}Overlay.

In `SceneInteractor`, the functions `snapToScene`,
`instantlyShowOverlay`, and `instantlyHideOverlay` remain as convenience
APIs, delegating to the consolidated function in the repository.

Bug: 411400348
Bug: 359173565
Test: Unit tests still pass.
Flag: com.android.systemui.scene_container
Change-Id: I6a3858317be26727ed88ef83d435df3f6bff5943