Make `scene_container` flag Desktop aware

- Creates a new flag named `scene_container_desktop` to enter long staging
- This will eventually let scene_container_desktop force enable scene_container


Bug: 433302027
Flag: com.android.systemui.scene_container
Flag: com.android.systemui.scene_container_desktop
Change-Id: Ib73d439114d9ba1b795ce3b7d67cf6b22e9585fe