Adding system support for a single accessibility focus.

Now that we have APIs to query all interactive windows and allow
an accessibility service to put accessibility focus in each of
them we have to guarantee that there is a single accessibility
focus. This is required for correct operation of the touch
explorer as on double tap in clicks in the center of the focused
area, hence having more that one focus is an issue. Also the
system is maintaining a single input focus so now accessibility
focus behaves consistently with that.

bug:13965563

Change-Id: I0b5c26dadfabbf80dbed8dc4602073aa575ac179