Bug 884517 - Change lockscreen to not collapse button area when screenreader is enabled.

Updates:
 - Conditionally remove 'triggered' class when screen reader is disabled.
 - Also deleted some obsolete 'triggered' class removal lines that followed.
Bug 884514 - Hide windows when lockscreen is active.

Changing the style of the app iframes won't work well because it will interfere with the way that audio apps work. So we need to use aria-hidden.
Merge pull request #10469 from eeejay/lockscreen-a11y-tweaks

Bug 884514, 884517 - Lockscreen a11y tweaks