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
Just  a note that if dismiss the screen by pressing home, the original notification create by message could not display the screen again if user tap on it because window is still opend at that moment and system message will not dispatched to opened window