[flexiglass] ShadeHeaderViewModel is a SysUiViewModel

As per go/sysui-arch:summer-2024

Bug: 354270224
Test: unit tests still pass
Test: manually verified that the shade header works in flexiglass as I
transition between shade and QS scenes
Flag: com.android.systemui.scene_container

Change-Id: I865cf0812c9686291f97e2c3fce35d32890792f7