[System.Core/Android] Use correct comparison for timezone ids.

Context: https://bugzilla.xamarin.com/show_bug.cgi?id=7953

The names within zoneinfo.idx are sorted ordinally, which is not the
default string comparison that Array.BinarySearch() uses.
Consequently, many timezones would not be found even though they were
present, including "Pacific/Auckland".

Use the correct string comparison so that names can be found.