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 thought, as with this patch ```sendAlert``` is only called if CMAS setting is enabled, having ```opts.title === CMAS_ID``` check inside it started to look a bit confusing. Moreover we request settings for every cell broadcast message. Maybe we can just simplify it for now and narrow down our support to CMAS only and extend only if needed? Eg. (it basically looks like a candidate for single ~25 lines function, but separate comments on function level still look better )
````
function isEmergencyAlert(id) {
  return id >= 4370 && id < 4400;
}

function onCellbroadcast(message) {
  if (isEmergencyAlert(message)) {
    window.close();
    return;
  }

  var req = navigator.mozSettings.createLock().get(CMAS_ENABLED_KEY);

  req.onsuccess = function() {
    if (req.result[CMAS_ENABLED_KEY][message.serviceId]) {
      sendAlert(message);
    }

    window.close();
  };
  req.onerror = function() {
    console.error('CMAS: Unable to query settings database');
    window.close();
  };
}

function sendAlert(message) {
  var url = [
    'attention.html?title=',
    encodeURIComponent(CMAS_TITLE_L10N_ID),
    '&body=',
    encodeURIComponent(message.body)
  ].join('');

  window.open(url, '_blank', 'attention');
}
````