For the 1979–80 fiscal year and thereafter, except as provided by subdivision (b) of Section 12 of Article XIII of the Constitution, for purposes of computing tax rates on the unsecured tax roll, the county auditor may add to the 1-percent rate the rate levied on the prior year’s secured tax roll for indebtedness approved by the voters prior to July 1, 1978, and bonded indebtedness for the acquisition or improvement of real property approved by two-thirds of the voters on or after June 4, 1986, as described in subdivision (b) of Section 1 of Article XIII A of the California Constitution.