[flexiglass] Move headsUpInset from the NSSL to the composables

NSSL used to determine the y position at which HUNs should be displayed.
This is the status bar height, plus some padding. This is another
example for determining the drawing bounds of the NSSL, which should
come from the placeholders.

Fixes: 387458294
Test: post HUNs over GONE -> check if HUNs are positioned correctly
Flag: com.android.systemui.scene_container
Change-Id: Icbe83d4cbfff7b92925a4045da3a999fb0993451