ATLANTA — The Atlanta Falcons agreed to terms on a five-year, $47.5 million contract with three-time Pro Bowl center Alex Mack on Wednesday, addressing one of the team’s most pressing needs. The deal makes the longtime Cleveland Browns starter the NFL’s highest-paid center, with an average salary of $9.5 million. The Falcons also reached an agreement with quarterback Matt Schaub, who will be returning to the city where he launched his NFL career.