Bug 995035 - Hide invisible parts of SMS app from screen reader.

Just wondering, wouldn't it be better and more self-contained if we just define _container_ id for every panel indide navigation.js and don't touch other js files in this patch? IMO we may need something like this inside navigation itself when we start to lazy load panels anyway.

I'm not sure though if it runs counter to what @julienw had in mind while designing navigation.js. Maybe @steveck-chung knows. Steve, what approach do you think will be better to use here?

Eg. like:
```
panels: {
  'thread': {
    behaviour: 'ThreadUI',
    wrapperPosition: 'left',
    container: 'thread-messages'
  }
......
```
And call it like:
```
if (currentPanelInfo) {
  document.getElementById(currentPanelInfo.container).setAttribute(
    'aria-hidden', 'true'
  );
}
document.getElementById(nextPanelInfo.container).setAttribute(
  'aria-hidden', 'false'
);
```