Enabling accessibility focus only if explore by touch is on.

1. Now we will enable the accessibility focus only if Explore by
   Touch is enabled. Enabling this feature allows a blind user to
   touch the screen and set the accessibility focus at this
   location as well as get spoken feedback. Also an accessibility
   service can move the accessibility as a result of user gestures
   detected only if Explore by Touch is enabled. Since we will
   handle some gestures by default if not accessibility service
   does so to provide reasonable built-in navigation of the UI
   by "objects" we need the accessibility focus functionality.

bug:6383361

Change-Id: I13ce6072a90f5838c7656379788144c99a772bf0