Support notifications with 0 count (show as dots)

- Show number if number > 0
- Show icon if number == 0 and a notification specified an icon to show
- Show a dot otherwise
- In cases of multiple notifications, stack a second badge behind the
  first (visuals will be updated in future CL, as well as support
  stacked dots)
- Folders always show dot if any app within has a badge.

Change-Id: I0a89059b0e0a0d174fe739c9da4f75fa18c0edfa