Bug 918970 - display existing thread when sending a sms from contact app

Actually, both sender and receiver (or receivers) are filled in both received and sent messages. 

Checking that point, I've noticed something very strange about MozMobileManager. I added a logger in my 'each' callback, and I have this (only 4 last digits of msisdn are shown for privacy :smile:  ) :

```
E/GeckoConsole( 4388): Content JS LOG at app://sms.gaiamobile.org/js/activity_handler.js:119 in findThread: filtering with 8953
E/GeckoConsole( 4388): Content JS LOG at app://sms.gaiamobile.org/js/activity_handler.js:108 in checkCandidate: message : sender: 0907 receiver: 3856 delivery:sent type: sms
E/GeckoConsole( 4388): Content JS LOG at app://sms.gaiamobile.org/js/activity_handler.js:108 in checkCandidate: message : sender: 0907 receiver: 7977 delivery:sent type: sms
E/GeckoConsole( 4388): Content JS LOG at app://sms.gaiamobile.org/js/activity_handler.js:108 in checkCandidate: message : sender: 3856 receiver: 0907 delivery:received type: sms
E/GeckoConsole( 4388): Content JS LOG at app://sms.gaiamobile.org/js/activity_handler.js:108 in checkCandidate: message : sender: 0907 receiver: 3856 delivery:sent type: sms
E/GeckoConsole( 4388): Content JS LOG at app://sms.gaiamobile.org/js/activity_handler.js:108 in checkCandidate: message : sender: 0907 receiver: 7977 delivery:sent type: sms
```
And maybe a hundred more. I do have some messages that match 8953 (the number I'm filtering with), but most do not. Is it a bug? It actually makes the process a bit lengthy for the user, and I'm pretty sure it wasn't there like 3-4 months ago... What do you think?