Bug 985037 - Update a11y visiblity management in light of lockscreen changes.

We should not be setting aria-hidden on #windows anymore, instead it should be on individual apps.
Bug 985037 - Part 2, fix lockScreenWindow visibility state
Merge pull request #18518 from alivedise/bugzilla/985037_master/lockscreen-window-visibility

Bugzilla/985037 master/lockscreen window visibility, r=timdream