[Flexiglass] Notif scrim height matches screen height on QS

Make the Notification scrim be able to expand below the visible area on
the QuickSettingsScene, to ensure that the Notification stack has space
to be laid out, when it is pushed below the screen.

While ideally the Notification scrim wouldn't be composed at all on the
QS scene (as it is not displayed), this is the least risky solution to
match the legacy behaviour of the NSSL, and the QS -> Gone transition.

Bug: 433919013
Bug: 433910331
Test: test all the scenarios for navigate to/from QuickSettingsScene
Flag: com.android.systemui.scene_container

Change-Id: I3d82edfdb7b76b548adc96b828e1bb6e2c9a4403