Use MessageCache::getMsgFromNamespace() to fetch conversion tables.

After Iaaf6cceb, MessageCache::get() goes through the fallback chain,
which is unwanted for conversion tables (for example, we don't want
zh-hans table for zh, where zh means "no conversion"). Since the only
needed feature is to fetch text from MediaWiki namespace (conversion
tables in PHP are stored somewhere else), it is now changed to use
MessageCache::getMsgFromNamespace() instead to avoid fallbacks.

Change-Id: I46e0be31c9c0fe0a6e4923fc1aff0fbbadbf1d67
Merge "Use MessageCache::getMsgFromNamespace() to fetch conversion tables."