MINEOLA, N.Y. — For months, police trying to solve a Long Island robbery spree had little more to go on than grainy surveillance footage of a man in a hoodie and black ski mask holding up one gas station or convenience store after another.That was until the gunman made off with a stack of bills that investigators had secretly embedded with a GPS tracking device.Within days, a suspect accused of pulling off nearly a dozen heists — including one in which a clerk was killed — was behind bars, and officers were crediting …