Bug 1074123 - Make modal dialogs use layout manager, remove SHB css work...

...arounds
TLDR; #permission-screen is height of the screen, but #permission-dialog is the element we want here because it houses the permission prompt more directly than #permission-screen. #permission-screen is just a container, much like #dialog-overlay.

Sorry for the long explanation here.

It's true, we need to be really careful here. I spent a lot of time looking at all the dialogs across the system app and decided to try and unify the different dialogs somewhat. There are too many overlapping selectors that apply to very different DOM scenarios, and so fixing little bugs was starting to require more and more specific css selectors to override any erroneous general selectors. So, the two big changes I made are: 1) that modal dialogs now use the layout manager to set dialog container heights (like system dialogs always have). This should fix the problems we have for fullscreen vs non-fullscreen dialogs. And 2) permission dialogs use the bottom property to set the height of the permission prompt wrt SHB. So now #permission-dialog is actually the element we care about in this test (and probably always have), since the entire permission prompt lives in it and it is what the bottom property applies to.