WASHINGTON, April 28 (Reuters) - Canadian Prime Minister Justin Trudeau on Friday linked the production of lithium in China to "slave labor" as he discussed his own country's efforts to ramp up production of the metal used in electric vehicle and other batteries. Canada has significant sources of lithium, Trudeau said, but China has made strategic choices over the decades that have made it by far the world's largest producer. "If we're honest ... the lithium produced in Canada is going to be more expensive. Because we don't use slave labor," Trudeau said in remarks at the Council on Foreign...