[flexiglass] Preserve notif composable state across scenes

Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Test:
  1. Go to shade
  2. Scroll notifications
  3. Tap a notification to go to bouncer
  4. Return to shade, observe scroll state is preserved

Change-Id: I9808a02d563ac406a6f15e896af669f7cae03a69