Add ranking object to each NotificationEntry

- Attach the Ranking object directly to NotifEntries (for use in the
future instead of derived properties).
- Add getters for key(), sbn(), and ranking() to allow us to spin off an
interface in the future.
- Add a loophole for tests to allow them to continue passing null
rankings for now. We should get rid of these long-term.

Test: atest
Change-Id: I4d28bfcd1851b4e67f165a34557787512234f89f