Existing calls to `ThreadUI.setMessageBody(value)` (they only exist in message_manager.js) will need to be updated:

- https://github.com/mozilla-b2g/gaia/blob/master/apps/sms/js/message_manager.js#L253 
- https://github.com/mozilla-b2g/gaia/blob/master/apps/sms/js/message_manager.js#L265
```js
ThreadUI.setMessageBody(activity.body); => Compose.fromMessage(activity);
```
For the sake of clarity, `Compose.fromActivity` could be an alias to `Compose.fromMessage`, which would make the above line even more understandable: 

```js
ThreadUI.setMessageBody(activity.body); => Compose.fromActivity(activity);
```

And this: https://github.com/mozilla-b2g/gaia/blob/master/apps/sms/js/message_manager.js#L197-L215 will just be removed and replaced with `Compose.fromMessage(message)`