[flexiglass] Expose the new snapToScene API.

ag/27208377 exposes a new snapToScene API in
MutableSceneTransitionLayoutState. This CL exposes that throughout the
Flexiglass scene framework APIs such that users of SceneInteractor can
now call snapToScene in the same way as they can already call
changeScene.

Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Bug: 330672236
Test: added unit tests for new functionality
Test: manually tested with follow up CL
Change-Id: I23def24c8fbb95048ea8c512d50cd9d177ecebd9