Cops have arrested a Brooklyn teen who they think is the serial subway robber who’s been ripping off people all over Manhattan since February. First he stalks his prey. Then he smiles at them and chats them up, either on the street or subway platforms and corridors. Then he displays a gun or knife and steals their cells and other property. At least six times in all. Cops released a video of a suspect a week ago. Last night, April 13, Devante Scott, 17, of Canarsie, was busted, charged with robbery and menacing.