Settings crashing if there are no installed accessibility services.

If there is no installed accessibility services and the user goes into
the accessibility settings section, the app crashes. This is due to a
lacking null pointer check against the list of installed services
returned by the accessibility manager. We should not have returned
null, rather an empty list - too late now, sigh...

bug:8871034

Change-Id: Id3800d398af83868862847fa7ed861a9dee61c8f