[flexiglass] legacy flag assert in NSSLC#setHeadsUpBoundaries

Assertion is already present in the NSSL, this just adds some clarity.

Flag: com.android.systemui.scene_container DEVELOPMENT
Fixes: 347998600
Test: make
Change-Id: I7800413aaa47d7e5edbe752c7f6490c8a4b4a349
1 file changed