[Flexiglass] Determine the z position of HUNs with the SceneContainer stack

 - when not in shade, set them to the default elevation, assume that we
   are not stacking HUNs on top of each other
 - when in the shade, set the elevation based on the overlap with the
   scrolling stack content

Bug: 339181697
Test: check if HUNs are displayed correctly in Gone, Shade, and QuickSettings
Test: atest StackScrollAlgorithmTest
Flag: com.android.systemui.scene_container
Change-Id: I7572b2199b1b17bff4e605ea127f747b8acb7430