A young man accused with others in the shooting death of a 16-year-old Moreno Valley boy was charged Tuesday with firearm assault. Avon Orville Lee Owens, 18, of Moreno Valley, was arrested Friday following a three-month-long Riverside County Sheriff’s Department investigation into the slaying of the teenager, identified in court documents only as “John Doe.” Owens, who is being held in lieu of $30,000 bail at the Robert Presley Jail, was slated…