WASHINGTON — Treasury Secretary Jacob Lew has decided to put Harriet Tubman on the $20 bill, making her the first woman on U.S. paper currency in 100 years, a Treasury official said today.The official, who spoke on condition of anonymity in advance of Lew’s official announcement, said that the 19th century abolitionist and a leader of the Underground Railroad would replace the portrait of Andrew Jackson, the nation’s seventh president.Lew’s announcement is expected to provide details on other changes being made to the $20, …