[flexiglass] Switch to Gone during LS transition

WindowManagerLockscreenVisibilityInteractor determines when the surface
underneath lockscreen is made visible; SceneContainerStartable now
tracks this and forces the scene to Gone when true so that Lockscreen
isn't rendered above the surface.

Flag: com.android.systemui.scene_container DEVELOPMENT
Fixes: 344844978
Test: manual
  0. Enable flexiglass
  1. Swipe up slightly on lockscreen, do *not* fling
  2. Lift finger
  Observe: Lockscreen is not visible over launcher
Change-Id: I9a6645c74c128373966b62e6aab08f8ef86ea16b