A 22-year-old central Illinois man who was arrested and detained after posting online pictures of himself burning an American flag has filed a federal lawsuit seeking to have the state’s flag desecration law declared unconstitutional.Such state laws are already invalid after the U.S. Supreme Court ruled separately, in 1989 and 1990, that flag burning and other forms of damage are constitutionally protected free speech. However, dozens of states still have the laws.Police in Urbana used Illinois’ flag …