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: there are also other ways to set mock settings in advance:
```
MockNavigatorSettings.mSet({
  'cmas.enabled': {
    0: true,
    1: false
  }
});
```
or
```
MockNavigatorSettings.mSettings['cmas.enabled'] = {
  0: true,
  1: false
};
```