The average price of a gallon of self-serve regular gasoline in the Inland Empire dropped one-tenth of a cent today to $3.956, one day after a 17-day streak of decreases ended with a half-cent increase.  The average price in Riverside and San Bernardino counties is 2 cents less than one week ago, 5 cents lower than a month ago and 13.5 cents below what it was one year ago, according to figures from the AAA and Oil Price Information Service.