One of the last survivors of Oakland’s notorious drug wars, 34-year-old Sonnie Davis, has been convicted in federal court of conspiring to traffic in heroin and of obstructing justice. He faces a maximum sentence of life in prison. Convicted with Davis was his nephew, Ken Davis, 27. Both men were tried in U.S. District Court under a federal program designed to enhance penalties against major drug dealers. They will be sentenced May 24. Alameda County Assistant Dist. Atty. Russ Giuntini, the appointed prosecutor in the case, said Davis’ drug-dealing career spanned shoot-outs with rivals going back to the 1970s. Giuntini said that while in jail, Davis ordered the murder of a woman he feared would testify against him.