CurrentSystemTimeZone.GetUtcOffset now supports multiple DST periods.

DateTime.Now was returning wrong values when system TimeZone had more than one daylight saving time.
**Fixes:** [22648](https://bugzilla.xamarin.com/show_bug.cgi?id=22648).
**Fixes:** [23170](https://bugzilla.xamarin.com/show_bug.cgi?id=23170).