[Flexiglass] add assertInLegacyMode for unused NSSL methods

The responsibility of placing the Notifications is moving out of the
NSSL in Flexiglass, so these methods are not functional anymore.
They were called by the NPVC or the QuickSettingsControllerImpl.

Bug: 332574413
Test: show Notifications with Flexiglass on
Flag: com.android.systemui.scene_container
Change-Id: I451b94b82b634c12a54cf3c25b624c0d6fa97c37