WASHINGTON (AP) — One lesson from Julia Pierson’s short tenure as director of the agency that protects the first family: The Secret Service can’t keep secrets from the president.Pierson’s undoing was not telling the president about a Sept. 16 incident in Atlanta in which President Barack Obama rode an elevator with an armed security contractor during a visit to the Centers for Disease Control and Prevention, two White House officials said. The armed contractor’s proximity to Obama violated the agency’s security protocols.Homeland Security Secretary Jeh Johnson, as well as Obama, appeared unaware of the full extent of the Atlanta incident and the Sept. 19 security breach in which a man armed with a knife jumped the White House fence and entered the building. Johnson, whose department oversees the Secret Service, was the driving force behind Pierson’s resignation Wednesday, the officials said.Since taking office last year Johnson had made it clear he wouldn’t tolerate even a whiff of scandal.