[Flexiglass] Verify isBelowLastNotification unused

Add an assertion to NSSL.isBelowLastNotification, because it was only used for the empty space logic.

Bug: 332574413
Test: verify that the new assertion is passing
Flag: com.android.systemui.scene_container
Change-Id: Ia513c4b320383e9a393210ea266d61c2458b8b01
2 files changed