NEW YORK — After weeks of plugging increasingly inviting discounts, retailers in the United States and several other countries are preparing for prime time: Black Friday, the bargain bonanza that still reigns as the unofficial kickoff of the holiday shopping season even if it’s lost some luster. Department stores, shopping malls and merchants — big and small — see the day after Thanksgiving as a way to energize shoppers and to get them into physical stores at a time when many gift-seekers are content to do their browsing online.