ShadeLockscreenInteractor: change scene on main thread

If we do this on the background thread, we trip a new assert in
SceneTransitionLayout, so do it on the main thread instead.

Bug: 342424366
Test: manual: set lockscreen, post notif, go to lockscreen, expand notif
Flag: com.android.systemui.scene_container
Change-Id: Ib118764d1ff21f46560dee9a7e914325d74eab96