CHICAGO — The white Chicago officer who gunned down a black teenager in 2014 was sentenced Friday to nearly seven years in prison, ending an explosive case that arose from one of the nation’s most graphic dashcam videos and added fuel to debates about race and policing and law enforcement’s “code of silence.” Jason Van Dyke was convicted last year of second-degree murder and 16 counts of aggravated battery — one for each bullet fired at Laquan McDonald.