Chris Rock had it all wrong, as it turned out, when he once laid some cynical conventional wisdom about show business on Jack Johnson . In December 2003, a 28-year-old Johnson found his way onto “The Late Show with David Letterman.” A few years prior, the Oahu native had been chasing a future as a professional surfer; now he was a bona fide music star whose second album, “On and On,” had just crashed into the top 10 of the Billboard 200. Waiting backstage with Johnson were his wife, Kim Baker, and best friend, Emmett Malloy , where they shared a memorable encounter with Rock. Recalls Johnson, “We were in this little dressing room and I said, ‘Oh, this is Emmett, and I’m Jack. We’re best friends. He’s my manager, too.’ He looked at us and said, ‘ Ah, you guys blew it. You guys are going to hate each other in about five years.…