Add legacy mode assertions for the scene container flag

Methods that were only called by legacy shade classes (i.e. NPVC and
QSC) now have a legacy mode assertion. Some methods were also used
internally by NSSL, and in those cases I added a private internal
version of the method with no assertion.

Fixes: 348013571
Test: manually verified with and without the flag enabled
Flag: com.android.systemui.scene_container
Change-Id: Ib20e9a25f04c7ec854b477c1a3a92b7842ba1ba0