Confess: The Autobiography,” out Tuesday — that California cops covered up for his long-ago arrest for “public indecency” at a Venice Beach restroom.In a book excerpt from Rolling Stone, the heavy-metal legend also relates a tasty affair from the early 1990s, when a then-closeted Halford would head to Camp Pendleton in California for threeways with a bisexual Marine sergeant and his wife — the only woman Halford says he has ever had sex with.“Once the door was shut, we would go at it for hours,”