Clean-ups and simplifications in KCalendarSystem to help in the new Locale KCM.

Add new static api to convert calendar type to calendar system enum as I was
copying this to 3 or 4 other places.

Move era config into a sub-group of Locale rather than as a top-level group to
make the kcm easier, these config options have never been saved before so this
doesn't break anything.

All tests passing here.