Clear binder calling identity before mutating secure settings.

The accessibility manager service changes secure settings without
clearing binder calling identnty. Luckily it worked for a long
time as the calls resulting in a secure setting change were not
made on a binder thread. Now we clear calling identity before
mutating secure settings - note that we alsways do this after
a security check, hece it is safe.

bug:26766532

Change-Id: Icb188e6de1a134f38de0bdb23c67fa4ff2806ba4