SystemUI: Fix notification header disappearing on tablets

For some reason, the notification panel header has weird dimensions
when it has anything else than its original ColorDrawable as
background, and only on tablets.

We workaround this by not changing the drawable until requested.

Change-Id: Id76a5dcacf59c35b2e49681d6eeb4e828f7cbc58
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: Ife399b82a3f272fb2ecc8e738c5f274f7682a556