Fix UserManagerService.setUserIcon throwing expt.

setUserIcon sends a broadcast that requires INTERACT_ACROSS_USERS.
Since the function directly checks MANAGE_USERS permission, added
a clear calling identity.

Alternative solution would be to make function check for one of
INTERACT_ACROSS_USERS permissions before performing any actions.

Change-Id: Ibbf27d4e188d059671024e4bb62f92a2daea3661