ADDIS ABABA, Ethiopia — President Obama asserted on Tuesday that he could win a third term if allowed by the Constitution, but said he was looking forward to life after the presidency, when he will be able to take a walk and spend more time with his family. As he wrapped up what may be his final trip to Africa while in office, Mr. Obama took aim at some of the continent’s gerontocracy and called on long-entrenched leaders to step down, declaring that “nobody should be president for life.” But it led to an off-the-cuff riff about his own improved...