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
Yeah, I forgot the sms permission at first, but I think of this and add it right after I ping you . For the "Tap to show" behavior, I think it works differently than normal notification call path we handled.  BTW I still have to create a follow up bug because the system tray will keep the original notification we created and new "Tap to show" notification at the same time. We will need to keep just one of them, and Jenny might prefer we always display the normal notification when user press home as we discussed the possible changes before, but l thin we could address this in another bug because we need spec updated for it first.