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
I noticed that if you receive first cmas, press Home button, then receive second one, press Home button. Then tap on the top bar to show the latest alert, dismiss it - everything still fine. Then when you go to the notification tray and tap on "Tap to show" then our emergency alert has ```window.opener === null```. Not sure if it's a bug, but I'd rather do smth like this:
```
window.opener ? window.opener.close() : window.close();
```