SystemUI: revert notification panel drawables, seperate brightness layout

after these custom notification panel commits, we also changed the drawables
of the panel background to black / really dark.

lets restore the previous drawables.

also seperate the brightness slider layout file, so we dont have a dependency
on the quicktiles brightness dialog.
also change the background of it, so it wont get transparent if enabled.

Change-Id: Ie9b4f6dc9811fe9c42119cda0221cf5f8d9d8311