WASHINGTON — Bestselling author Stephen King gingerly stepped up to the witness stand Tuesday in a federal antitrust trial and, tracing his own history, laid out a portrait of a publishing industry that has become increasingly concentrated over the years while richly rewarding his creative endeavors.“My name is Stephen King. I’m a freelance writer,” King said as he began his testimony as a witness for the U.S. Justice Department. The government is bidding to convince a federal judge that the …