Assert legacy mode in 3 NSSL methods that register callbacks

None of setOnScrollListener, setOnStackYChanged, or
setOverscrollTopChangedListener should be called when scenes are
enabled.

Fixes: 348015461
Test: manually verified non-crash with scenes enabled
Flag: com.android.systemui.scene_container
Change-Id: Iae958070a7d2d11ae2a3ee9eb6c60587951ff7a4