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

old PR : https://github.com/mozilla-b2g/gaia/pull/16216

same here:
```javascrit
switch(message.type) {
  case 'sms':
    return Utils.probablyMatches...
  case 'mms':
    return message.receivers.length === 1 && Utils.probablyMatches(...);
  default:
    console.error('Got an unknown message type: ', message.type);
    return false; // just to be sure
}
```