[flexiglass] Improved structured concurrency in SceneContainer.

In an effort to improve performance by leveraging structured
concurrency:

1. Changed Scene.destinationScenes from StateFlow to just Flow
2. Changed SceneContainer to collect only the destinationScenes of the
   current scene
3. Added Scene.activate to hydrate scenes
4. Changed SceneContainer to call activate on the current scene and
   cancel the one that was previously activated
5. Removed @Application CoroutineScope from scene view-model classes
   that didn't need it anymore (more cleanup to come)

Bug: 354270224
Test: manually ran a smoke test on Flexiglass: can change scenes to
shade, QS, and bouncer over lockscreen, can unlock, can change to shade
and QS, and re-lock.
Flag: com.android.systemui.scene_container

Change-Id: I3dfee72630322e0cb93ac76324be9d935e3a67f1