ATLANTA — America has honored Martin Luther King Jr. with a federal holiday for nearly four decades yet still hasn’t fully embraced and acted on the lessons from the slain civil rights leader, his youngest daughter said Monday. The Rev. Bernice King, who leads The King Center in Atlanta, said leaders — especially politicians — too often cheapen her father’s legacy into a “comfortable and convenient King” offering easy platitudes. The service, sponsored by the center and held at Ebenezer annually, headlined observances of the 38th federal King holiday.