The father of a mentally disturbed teenager who fatally shot two Fairfax County police officers last year was sentenced to more than three years in prison yesterday for illegal weapons possession. Brian H. Kennedy, 50, of Centreville, pleaded guilty in August to weapons charges in U.S. District Court in Alexandria as part of a plea bargain. Kennedy admitted that he illegally purchased an AK-47 rifle by failing to disclose on federal background checks that he regularly used marijuana. His son, Michael, used the AK-47 in the May 2006 ambush that killed Detective Vicki O. Armel and Officer Michael E. Garbarino....