Changes filtered conversation thread database query to account for group...

...s

Fixes #1954

I go into discussion on this fix (and an alternative) in some depth in the issues ticket: 
https://github.com/WhisperSystems/TextSecure/issues/1954

I've tested this fix on Motorola Atrix 2 running 4.0.4, verified first that searching doesn't show groups, and then that after this fix they show up in a search. 

The crux of this is bug is that the thread db search uses an equals operator to query and individual recipientIds are a substring in the case of a group recipientId, for which equals does not match.