Refactor notification to allow for overlay by Android Auto.

Android Auto requires different styling for notifications. In
particular:

- Background of scrim to be a different color
- Notification shade should always be full width
- No quick settings
- User switcher always visible
- Card styling, such as rounded corners and more spacing between items.
- Notification shade to be triggered from a button on the navigation
  bar.

Refactored different areas of the framework to allow for this. This
mostly means extending notification code and overriding methods where
appropriate.

Test: manually tested
Bug: 33487455
Change-Id: Ifcea70e121c1cbbeb057bb6ce8d351336ae41554