Bug 1071510 - [NFC/sharing] Use shirinking* events to handle screenshot in AppWindow, r=alive

https://bugzilla.mozilla.org/show_bug.cgi?id=1071510
I'll retest this in couple of minutes and double check with WebIDE. I thought that _hideScreenshotOverlay is necessary because of this comment: https://github.com/mozilla-b2g/gaia/blob/c1a1664f83273bebd38678adfd6c2a4575c37b1e/apps/system/js/app_window.js#L202