Two titans — the U.S. Department of Justice and telecommunications giant AT&T — are locked in a high-stakes showdown to decide who controls some of the nation’s most popular television channels.The Justice Department sued to block AT&T’s planned $85 billion purchase of Time Warner, the New York media company that owns HBO, CNN, TNT, TBS, Cartoon Network and Hollywood’s largest movie and TV studio, Warner Bros.The dispute — a rare standoff in an antitrust case — will be decided by a federal judge after a trial that begins Monday in Washington, D.C., barring a last-minute settlement.The government alleges that AT&T, which already owns the nation’s largest pay-TV provider, DirecTV, would use its added clout to bully others, freeze out new entrants in the TV industry and increase rates for consumers.