WASHINGTON — Amy Coney Barrett was formally sworn in Tuesday as the U.S. Supreme Court’s ninth justice, her oath administered by Chief Justice John Roberts. Her first votes on the court could include two big topics affecting the man who appointed her.The court is weighing a plea from President Donald Trump to prevent the Manhattan district attorney from acquiring his tax returns. It is also considering appeals from the Trump campaign and Republicans to shorten the deadline for receiving and counting absentee …