[flexiglass] Verify flexi only NSSL methods are not called in legacy

Add SceneContainerFlag.isUnexpectedlyInLegacyMode() checks to every
public NSSL method that should be exclusive for flexiglass.

Bug: 296118689
Test: run sysui with flexiglass off -> make sure no
isUnexpectedlyInLegacyMode() checks are hit
Flag: com.android.systemui.scene_container

Change-Id: Id046ca42d2b23104881203caa24b1a27c4b510e4