[flexiglass] Update NSSL stack heights, when maxNotifications changes

AmbientState#mStackEndHeight depends on NSSL#mMaxDisplayedNotifications,
so we need to recalculate the stack end height, whenever the number of  max
displayed notifications changes.

Fixes: 365727076
Test: receive Notifications on the LS, check whether there is enough space
to be displayed
Flag: com.android.systemui.scene_container

Change-Id: Ia67a244b4c229d979718af528a1149f41eb8eee3