Bug 1112135 - [Messages] When starting as an activity/notification/, delay threads rendering until the right panel is open. r=julien

Can't think of better way than ```Promise.resolve``` hack :( Was thinking about Promise result as handleMessageNotification result as in snippet below, but it seems a bit misleading since promise will be resolved before ActivityHandler.toView is really completed, but what do you think?
```
handleMessageNotification: function ah_handleMessageNotification(message) {
  .....
  return MessageManager.getMessage(message.id).then((message) => {
    ......

    ActivityHandler.toView(message);
  }));
}
```