[Flexiglass] Introduce Topmost Content for STL x KTF

Exposes new topmostContent API through SceneInteractor that represent the highest z-order ContentKeys for a given STL state. This should help us avoid inconsistensies between STL and KTF state that are introduced by modeling certain states as Overlays within STL (namely Bouncer)

Bug: 390503464
Test: updates to existing tests
Flag: com.android.systemui.scene_container
Change-Id: Ie5cb16a95cd9cf9b412b6dfc34b3e6b59b1a4269