NEW YORK — On Monday, AT&T squares off against the federal government in a trial that could shape how you get — and how much you pay for — streaming TV and movies. AT&T says it needs to gobble up Time Warner if it’s to have a chance against the likes of Amazon, Netflix and Google in the rapidly evolving world of video entertainment. The Justice Department’s antitrust lawyers say that if AT&T and Time Warner are allowed to combine, consumers will end up paying more to watch their favorite shows, whether on a TV screen, smartphone or tablet. “On one hand, the government is saying this is the Old World and AT&T Time Warner is saying this is the New World,” said Larry Downes, senior industry and innovation fellow at Georgetown University.