[flexiglass] Remove double footer height from the stack height

Bug: 380210512
Test: check the bottom padding of the stack in dual shade
	- empty shade (no notifications)
	- few notifications (not enough to have a full height shade)
	- lots of notifications (enough to scroll the shade)
	- check in the shade
	- check in locked shade
	- check in the lockscreen
Flag: com.android.systemui.scene_container
Change-Id: I016c0ace9cee5fae24e12b8149050dc054b8190a