Remove NotificationData

Ye olde NotificationData class was responsible for a few general things:
- Keep a list of visible `NotificationEntry`s
- Filter / sort the visible entries
- Keep a sorted / filtered list of those entries
- Answer lots of questions about the entries themselves (rank,
isAmbient, list of entries for current user, etc.)
- Keep track of the current RankingMap
- Set priority buckets on entries
- Tell the group manager when things changed

The basic idea here is to remove NotificationData in favor of 3 other
changes:

1. Any place which needed to query NotificationData for info about the
entry (in particular its ranking) can just ask the entry for it now.
Entries all keep a reference to their rank

2. NotificationEntryManager now just maintains its own list of visible
notifications. It was already the point of contact to get a handle to
NotificationData so this makes call sites simpler.

3. Create a simpler NotificationRankingManager (maybe delete this
eventually) to encapsulate the sorting logic and hang on to the latest
RankingMap

Test: atest SystemUITests
Change-Id: I14073e103c6d50da37207ca8fd86828da0aaaff9