SystemUI: Cleanup and enhance notification drawer header

- Crop properly images on tablets and landscale
- Use a separate layer instead of breaking header background
- Fix bug where you couldn't turn header back on after turning it off
- Added a gradient overlay

Change-Id: Ic1c1de58018ae6927e65711c90d9e4c08b93b7e4
Signed-off-by: Alexander Martinz <eviscerationls@gmail.com>