DateTimeZone.UtcId should probably not be public

[GoogleCodeExporter]

We've defined the UTC timezone ID as "UTC".  It seems odd to provide a public 
constant for that as well, since it's not as if we can change it.

DateTimeZone.UtcId is used by e.g. FixedDateTimeZone, so it should probably be 
retained, but internal (or, better, protected internal, so that someone can 
easily create a 'new' UTC timezone).

Original issue reported on code.google.com by malcolm.rowe on 18 Apr 2012 at 9:34