Update notification visuals (part 2)

- Background is now white, and color beneath and divider color
  updated accordingly (not from color extraction)
- Removed overflow text ("+6") and added it to a header
  ("6 Notifications"). Use "..." instead if there is an overflow.
- Even spaced out icons in notification footer between the
  far right icon and the ellipsis
- Remove code to change arrow tint, since it is always white
  now. This also fixes the issue where it was drawn as a rect.

Bug: 35766387
Change-Id: I03bfda4ff029f23dd8b3dd1b72f534ea0e2c0816