WASHINGTON — The Justice Department is opening a sweeping investigation into policing practices in Minneapolis after a former officer was convicted in the killing of George Floyd there, Attorney General Merrick Garland announced Wednesday.The announcement comes a day after former officer Derek Chauvin was found guilty of murder and manslaughter in Floyd’s death last May, setting off a wave of relief but also sadness across the country. The death prompted months of mass protests against policing and the treatment of Black …