Correctly apply DST offset if the date being converted falls after Jan 1st and is within the last year's DST zone

The adjustment rule must take into account that DST can start in the previous year and extend to some initial period
of the DateTime's year. This patch makes the code take that into account and correctly applies the DST offset to the
date being converted to some target timezone.

Fixes xambug https://bugzilla.xamarin.com/show_bug.cgi?id=17155