Iraqi Prime Minister Adel Abdul-Mahdi said he would submit his resignation in response to criticism from the country’s top cleric following weeks of deadly protests, signaling a period of political instability for a fragile democracy caught between U.S. and Iranian interests. Fifteen people were killed Friday when Iraqi security forces fired tear gas and live ammunition to stop them from storming the police headquarters in the city of Nassiriyah, bringing the death toll to more than 350 since demonstrations began nearly…