[CS] 3/4: Move back gesture callback registration to back interactor.

Bug: 296050180
Test: open QS customization -> do back gesture -> verify customization
is closed (verified with flag on and off)
Test: open full QS -> do back gesture -> verify shade updates to be QQS
(verified with flag on and off)
Test: atest BackActionInteractorTest CentralSurfacesImplTest

Change-Id: I0c393b96d9dbed5e6d8de7583f9f03799f1e2ed5