Split some Language methods to LanguageNameUtils

These are static methods that have to do with processing language names
and codes. I didn't include fallback behavior, because that would mean a
circular dependency with LocalisationCache.

In the new class, I renamed AS_AUTONYMS to AUTONYMS, and added a class
constant DEFINED for 'mw' to match the existing SUPPORTED and ALL. I
also renamed fetchLanguageName(s) to getLanguageName(s).

There is 100% test coverage for the code in the new class.

This was previously committed as 2e52f48c2ed and reverted because it
depended on e4468a1d6b6, which had to be reverted for performance
issues. There should be no changes other than rebasing.

Bug: T201405
Change-Id: Ifa346c8a92bf1eb57dc5e79458b32b7b26f1ee8a
(cherry picked from commit 6d80b6c0827401cf8e41589bf134147bb0aa407f)