For several years, cellphone theft has been a growing epidemic worldwide. But the police in New York, San Francisco and London say they are finally starting to see a dip in thefts of one of the most popular smartphones. The reason? The attorney general of New York, Eric T. Schneiderman, and the district attorney of San Francisco, George Gascón, share the theory that phone thefts are dropping because of Apple’s addition of a so-called kill switch, a type of antitheft technology, in its iPhone in September. Police officers in San Francisco and London say that after Apple introduced its measure, called Activation Lock, last fall, iPhone theft fell significantly. Comparing data in the six months before and after Apple released the feature, the police said iPhone robberies in San Francisco dropped 38 percent, and those in London fell 24 percent.