[flexiglass] Lockscreen 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 lockscreen can navigate to adjacent scenes
and that its elements seem to animate and render correctly

Change-Id: Id677ab4d7d39652c7a10d48c39971572e47766eb