[Flexiglass] Flip a flag assertion in NSSLC

NSSLC#setMaxDisplayedNotifications is an API that we don't want to use
with SceneContainer anymore, because we are binding this property to the
view by its binder directly.
The previous refactor has introduced a WRONG flag assertion, and this CL
corrects it.

Bug: 421524579
Test: Test the flag assertion with flexi on- and off.
Flag: com.android.systemui.scene_container
Change-Id: I1bbeeb9b852e4f84fb79705fa2d8c1bdbf3a5aab