Apple Inc. Chief Operating Officer Jeff Williams is retiring as the company’s longtime No. 2, marking a major changing of the guard at an already tumultuous time for the iPhone maker.Williams will step down as COO this month before retiring from the company later in the year, Apple said in a statementBloomberg Terminal Tuesday. He will continue to oversee the design team, as well as manage health initiatives, until his departure. Sabih Khan will replace Williams as COO, while Apple’s design team...