[flexiglass] Move to prev scene on dismiss bouncer

We track the previous scene via SceneInteractor.previousScene, and
derive the BouncerScene#destinationScenes flow from it.

Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Fixes: 332967776
Test: manually verify that entering the bouncer from QS and then
dismissing returns to QS scene.

Change-Id: Ib58c628ead27be8af824276c00edb67420025167