Steve Jobs will forever be credited with inventing the iPhone. Tim Cook used it to build an empire. Cook, who built the iPhone-maker into a colossus with influence coursing through geopolitics, pop culture and the daily lives of billions of people, said he would step down as CEO on Sept. 1. He will remain as executive chairman. He is stepping away from the top job as Apple’s hero product surges to new sales records, its market capitalization exceeds $4 trillion and its revenue is more than $400 billion, quadruple what it was when he took over 15 years ago. Yet…