[Flexiglass] Fix back handling in QS Edit mode

When in edit mode, back should close edit mode. With this fix:
* Add a back handler in QuickSettingsScene. This is not needed in Split
  Shade as that behavior is not there.
* Remove the Back action in the destinations for Quick Settings. This
  would cause it to trigger first and then the back handler above not be
  considered.

There is a BackActionInteractor, but it doesn't seem to be working, just
in case fix the method used by QuickSettingsControllerSceneImpl to
properly close the customizer.

Test: manual
Test: atest QSSceneAdapterImplTest QuickSettingsSceneViewModelTest
Fixes: 330669681
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: I3dd51dcdd0f180ad074019ccaa1fb2931cf72c83