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

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

packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBar.java