BEIJING — China again signaled that Taiwan would be a priority topic ahead of a highly anticipated summit between U.S. President Donald Trump and Chinese President Xi Jinping next week, saying that U.S. must adhere to the “one China principle” for a stable relationship with Beijing. Last week , China’s top diplomat Wang Yi said he hopes the U.S. would make the “right choices” relating to the self-ruled island when he spoke with U.S. Secretary of…