Bug 1051795 - [CMAS] Handle the setting that enables/disables CMAS alerts

- Add settings permission and check the CMAS option before opening attention screen.
- Use Notification helper and mozApp to get icon url instead of using hard-coded string.
- Fix some error in Bug 1051793
nit: probably we can get rid of ```window.close()``` in ```sendAlert``` and call it here without ```else``` branch, so that we have exit points only inside this method:
```
if (req.result[CMAS_ENABLED_KEY][message.serviceId]) {
  sendAlert({
    title: getMessageTitle(message.messageId),
    body: message.body
  });
}
window.close();
```