Bug 1054126 - Support "fullscreen_layout" field in manifest

https://bugzilla.mozilla.org/show_bug.cgi?id=1054126
`isFullScreen` should also return true when `isFullScreenLayout` is true.
one of the goal of the `fullscreen_layout` property is to make sure the app frame won't need to be resized when requesting fullscreen. so we always want it going from top to bottom, with no status bar