Bug 1071235 - Fix touching fullscreen video displaying controls r=alive

CSS background-color in ".appWindow.collapsible .screenshot-overlay"
makes the platform breaking tapping on a fullscreen video. Setting
'visiblity: hidden' when we are fullscreen does workaround the issue.
Merge pull request #24693 from lissyx/bug1071235

Bug 1071235 - Fix touching fullscreen video displaying controls r=alive