There's not enough info either way to decide whether he's a robot or human. I'm leaning towards robot, personally, because robots aren't nearly ubiquitous enough in the Trek universe and I'd be surprised if J.J. didn't look at this as something to be addressed.
It's also possible it's a robot with a remote, human operator. A police department equivalent of an unmanned drone.
I'm not sure what to make of the modulation in the voice. There are voice synthesizers available today that sound more natural than that, so I'd expect 23rd Century robots to have very natural-sounding voices. If the cop's human, perhaps the modulation is there to protect the identity of off-duty cops ... like a superhero who wears a mask so people in his or her personal life aren't put at risk. I don't like the implications of that, however.