[Flexiglass] Send stackCutoff to from the placeholder to the NSSL

Send a value to the NSSL, which marks the bottom-most acceptable
y-position of the bottom of the notification stack / shelf / footer.

Bug: 332574413
Test: turn on debug lines, see where stackFold is drawn
Flag: com.android.systemui.scene_container
Change-Id: I165da6c15ae14826211477c30cbb0a8f2381ab90
5 files changed