[flexiglass] Recalculate NSSL children positions on stackCutoff changes

Stack bounds are sent to the NSSL via two values: stackTop, stackCutoff.
Previously these values were changing in tandem, but with Dual Shade the
Notification Shade height changes from "match_parent" to "wrap_content",
and it causes funny bugs, because we were not recalculating the NSSL
children's positions, which was leaving an invisible shelf stuck at the
previous stackCutoff, which was clipping Notifications.

Fixes: 383709370
Test: trigger stack height changes while the shade is open
	- have one notification
	- expand it -> collapse it -> expand it
	- check if the expanded layout is entirely visible
Flag: com.android.systemui.scene_container
Change-Id: Ie1f58bdd73bbbae0ed7ce9d7e5db8099396d1fc8