[CS] Remove CentralSurfaces#readyForKeyguardDone.

Have CameraGestureHelper notify SBKeyguardViewManager directly.

Also makes CameraGestureHelper a singleton, since it looks like it's
used as a singleton.

Bug: 277764509
Test: verified via logging that SBKVM is notified when camera gesture
occurs
Test: atest CameraGestureHelperTest

Change-Id: Id32432c23395fa812a71b6f6dcf296d3ae52ad00