MARTINEZ, Calif. — In the conclusion of Contra Costa County’s first case involving a policeman who was prosecuted for an on-duty shooting, ex-sheriff’s Deputy Andrew Hall was sentenced to six years in prison Friday for fatally shooting a mentally ill man during a slow-speed police chase in 2018. Hall was convicted last year of assault with a firearm and a gun enhancement in 33-year-old Laudemer Arboleda’s shooting death, but jurors couldn’t reach a verdict on a more serious manslaughter count. Though the shooting occurred in 2018, Hall wasn’t charged until 2021, after he’d been cleared by a sh…