The Demi Lovato vehicle is the latest show from a channel reluctant to let its adolescents grow up. How is there not an "Oh No You Di'nt!" awards show in real life? It exists on " Sonny With a Chance ," the new Demi Lovato vehicle on the Disney Channel (Sundays at 8 p.m.), and like many things on this sitcom, which sends up Young Hollywood in ways only Old Hollywood might appreciate, it's modern and funny and knowing and, ultimately, true.