bug 1044125 - Only capture screenshot and show overlay when edge gesture...

...s are in use or task manager is used. r=alive
No, I think we should broadcast the message via AppWindowManager.
So it seems APM already broadcast start message to the top most one, but we need to broadcast end message to everyone who is not active
https://github.com/mozilla-b2g/gaia/blob/master/apps/system/js/app_window_manager.js#L541