Adjust for DST properly in GregorianCalendar.add(int,int).

Use the adjustForZoneAndDaylightSavingsTime() added in change
196553 to handle computing the time in millis in the
add(int,int) method. The removed code was very confusing as it
relied on side effects of calls to setTimeInMillis() a lot.

Removed an unnecessary fieldMask parameter from the
adjustForZoneAndDaylightSavingsTime() method, it was duplicating
information provided by tzMask.

Bug: 26022884
Change-Id: I476112cad93642340ff23779fa1902cbd0fa8e25