Bug 1060727 - [CMAS] Duplicated notifications in system tray when dismiss the attention screen by tapping on home button

- Close app while home button clicked (title display to none and visibility hidden)
It looks like just calling ```window.opener``` works as expected only for single case (probably the most frequent) when we have only one child attention window. But in case we receive one emergency alert -> then second -> press Home for the last one (it goes to status bar), reading emergency alert that we have received earlier and at that moment timeout is fired for the emergency alert that we previously dismissed with Home button and both\all emergency alerts are closed (via window.opener.close).

Do you think the case I'm describing is very unlikely to happen? Otherwise we'll need to use inter window communication like:
In attention.js:
```
window.opener && window.opener.postMessage({
  type: 'attention-window-dismissed'
}, window.location.origin);
window.close
```
and in index.js\notification.js
```
window.addEventListener('message', function(e) {
  if (e.origin === window.location.origin && e.data.type === 'attention-window-dismissed') {
    !--attentionWindowCount && window.close();
  }
}, false);
```