[Flexiglass] Make NotificationStackCutoffGuideline a shared element

The cutoff guideline drives the shelf's position. Make it a shared
element to ensure a smooth transition of the shelf between scenes.

Bug: 330468684
Test: observe the shelf in the shade -> qs transitions
Test: scroll notifications into the shelf
Flag: com.android.systemui.scene_container
Change-Id: Iac78e46af9dbde3439db045278dc96b14377ea0f