[Flexiglass] Fold NotificationsHeadsUpRefactor flag into SceneContainerFlag

Convert all usages of the NotificationsHeadsUpRefactor flag into
SceneContainerFlag, because we want to roll them out together.
Make SceneContainerFlag depend on NotificationThrottleHun, because we
have just inherited this dependency from NotificationsHeadsUpRefactor.

Bug: 325936094
Test: manually verified that HUNs are displayed in Flexiglass
Flag: com.android.systemui.scene_container
Change-Id: I4fc0aaa6f0d68837e5d8003d78177887980d7889