Volume dialog: Add zen mode icons and notification access.

- Add icon above text to all three zen mode states, update text style.
- Remove zentoast.
- Update shared borderless rect background, masks now support shapes.
- Update size of volume stream icons.
- Ensure all volume icons are expressions of white.
- Make volume icons testable via new demo mode command.
- Add a divider + secondary icon to access the notification slider.
- Animate the transition when accessing notification slider.

Bug: 18206097
Bug: 16303068
Bug: 18102850
Change-Id: I5eb6f820dc317e89be272cc78f6c80ed969ad5e9