SAN FRANCISCO :Just days after putting on its annual devices and services showcase, Amazon is losing a vice president who helps oversee the division. Rob Williams, vice president of device software and services and a member of the elite internal group known as the S-team advising CEO Andy Jassy, left his post and will step down entirely from Amazon at year's end, according to a memo sent internally Thursday and reviewed by Reuters. Amazon confirmed the memo after a query from Reuters without