Bug 1003165 - Prevent reflow when transitioning to and from the homescreen.
* Disabling edge zones without causing a reflow.
* Making sure the appWindow is never without a transform to prevent a reflow..
* Making sure all app windows have will-change set to prevent reflows when going into edge mode.
* Prevent the homesearch bar from causing a weird animation during the first transition back to homescreen before getting stuck with .transitioning = true.
* Removing the pointer-events:none during app transition.
Merge pull request #19558 from etiennesegonzac/bug-1003165-homescreen-transition-reflows

Bug 1003165 - Prevent reflows when transitioning to and from the homescreen r=alive,kgrandon