[Flexiglass] Allow notifiction scrim clipping on DualShade

Except during the Lockscreen -> NotificationsShadeOverlay transition to
allow moving Notifications to from the LS to the Shade during the panel
expansion.

If there's no clipping, NSSL can draw anywhere on the screen, which
causes a bug where the scrolling stack covers/overlaps with the UMO and
the shade header.

Fixes: 388475946
Bug: 429110183
Test: Notifs visible during the Lockscreen -> NotificationsShadeOverlay transition
Test: overscrolled Notifs don't overlap with the UMO in DualShade
Test: overscrolled Notifs don't overlap with the click in DualShade
Flag: com.android.systemui.scene_container
Change-Id: I78074383d9602a07af82b62249777896b3a1ddd8