[flexiglass] Don't rely on currentScene/Overlays StateFlow when snapping

This is because in subsequent calls to instantlyTransitionTo(), the
current state as represented by the StateFlows is not guaranteed to be
up to date.

Bug: 430084116
Test: Unit tests still pass.
Flag: com.android.systemui.scene_container
Change-Id: I50b0469d253e360ae64c8730163ad7d8d272c57c