[flexiglass] Gone scene view-model is SysUiViewModel

Also, no longer injected into anywhere.

Bug: 354270224
Flag: com.android.systemui.scene_container
Test: new unit test, updated existing unit and integration tests
Test: manually verified that the gone scene can navigate from/to adjacent scenes
and that its elements seem to animate and render correctly

Change-Id: I8cef0fb51bc415ec10d7726c165619ccdceb2fb9