Fix StructXfrmLifetimeCurTest failure when default timezone is not GMT

Previously the calculation of the expected ADD_TIME implicitly depended
on the default timezone of the test device and thus might have
different values and cause test failures. This fix updates the test to
always use "GMT" to generate the ADD_TIME

Bug: 308011229
Test: StructXfrmLifetimeCurTest passing with default timezone being
      GMT, GMT-6, GMT-10
Change-Id: Ib88a253c1ecb09feeb298a04b847c4105651a710