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

Conflicts:
	packages/SystemUI/res/layout/status_bar_expanded_header.xml