Act utilitarians specifically don't believe in evaluating the overall consequences of a policy. Rule utilitarians do that. That is, in fact, the major difference between the two.
Good point, I phrased it poorly. Because of the effects of the specific action, I think an act utilitarian should still refuse to be mugged in this case.
The policy I was describing is just a mental shortcut, a part of adapting morality to human beings. See https://en.wikipedia.org/wiki/Two-level_utilitarianism for more in this direction.