Adding an opt-in mechanism for gesture detection in AccessibilityService.

1. An accessibility service has to explicitly opt in to be notified
   for gestures by the system. There is only one accessibility service
   that handles gestures and in case it does not handle a gesture
   the system performs default handling. This default handling ensures
   that we have gesture navigation even if no accessibility service
   would like to participate/customize the interaction model.

bug:5932640

Change-Id: Id8194293bd94097b455e9388b68134a45dc3b8fa