The Chinese firm ZTE said on Wednesday it had ceased “major operating activities” after the Trump administration banned the company last month from using components made in the United States. With manufacturing halted at the ZTE plant in Shenzhen, factory workers have been getting called in for training sessions every other day or so — a snooze, they say. The rest of the time, they loaf around in nearby dorms. Trading in the company’s shares has been suspended for weeks.