“Over-policing is driven in part by the law of supply and demand—police go where people ask them to go. To put it a little differently: Police don’t operate in a vacuum. They are paid by taxpayer dollars; they respond to the directives and incentives created by national, state, and municipal laws, policies, and political pressures; and in a day-to-day sense, they respond to whatever calls happen to come in over the 911 lines, whether those calls involve complaints about armed robberies or about disorderly conduct.”