Migrate dart SDK from `fuchsia.timezone` to `fuchsia.deprecatedtimezone` throughout.

`fuchsia.timezone` and `fuchsia.deprecatedtimezone` are going away.  They will be replaced by appropriate `fuchsia.intl.*`calls where needed.

As a first, and hopefully easy step in the migration, I'd like to remove one of them in the Fuchsia code base.  
Before that happens, though, we'll need to evict all uses of `fuchsia.timezone` where they appear.  Hence this bug ticket.