An off-the-cuff exchange between President Trump and China’s leader, Xi Jinping, on Friday provided a glimpse into how the two leaders interacted this week as they negotiated the rivalry between their countries. Mr. Xi, who is rarely shown in Chinese media without prepared remarks, was filmed by the U.S. press traveling with President Trump. The two leaders are seen chatting through translators as they walk through Zhongnanhai, a walled compound that houses China’s leaders. Mr. Xi puts his hand out to guide Mr. Trump through an archway, pointing to a…