It’s easy to interpret the first single from the Backstreet Boys’ upcoming album as a mea culpa for their prefab past (”Baby, please try to forgive me/ … I played my part”). Too bad the mannered, Max Martin produced ”Shape of My Heart” sounds as contrived as their earlier hits, yet not as playful and sweet as the salad days’ smash ”As Long as You Love Me.” This soggy mid-tempo ballad is more a joyless retread than a maturing makeover. C