Bug 1026575 - [B2G][SMS] Message preview in Messages app thread view disappears after opening app. r=schung

I see other (probably more correct) option to resolve this:
```
 afterLeave: function thui_afterLeave(args) {
   // We have this currently 
   if (Navigation.isCurrentPanel('thread-list')) {
      ......
      this.recipients.length = 0;
      .....
    }
    if (Navigation.isCurrentPanel('thread')) {
      this.recipients.length = 0;
    }
  }
``` 
But to be on the safe side I'd rather do it once we separate Thread and Composer views as well as unsubscribing from ```visibilitychange``` event in ```after\beforeLeave```.

What do you think?