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 didn't verify the error in promise reject because sendNotification function didn't expose for testing, but I'm not sure if it's a good move to expose it simply for testing. Do you think we should expose it to make it testable?