Some callers need the raw nul-terminated c-string in this callback, to
pass to some other ICU method. But other callers will want a StringView.
We know the length already in `icu_string_enumeration_to_list`, so let's
just pass it along to avoid `strlen`.
We were manually doing this for the calendar keyword, and would need to
do so for the collation keyword as well. I wasn't aware of this API
originally, so let's start using it.