[SMS Plugin] Return conversation results one-by-one to more quickly populate the GUI

Summary

Since !158 (merged), fetching messages has been painfully slow. This is because, instead of making one cursor (database) call, we needed to make two per conversation.

Unless someone finds a conversations URI which works on Samsung devices and reliably returns data, there isn't much we can do about that. What we can do, is return each conversation more quickly to get the UI populated and to prevent blocking the main thread forever.

This attempts to return conversations ordered by most-recent first. If the conversations URI returns junk for the thread date, it will return in an undefined order.

Test Plan

#itworksforme