Dramatically simplify NotificationRankingUpdate.

It is now simply a container for a parceled RankingMap,
which can be unparceled entirely into objects in the
listener's heap and discarded. Notification key strings are
interned (by reference into the Ranking data structure).

This should save a considerable amount of RAM:

 - parceled Bundles hanging around in the listener process,
   waiting to be unparceled, never freed
 - a dozen copies of the notification key string

Bug: 133763354
Test: atest NotificationManagerServiceTest NotificationListenerServiceTest NotificationDataTest
Test: coretests, cts, etc.
Change-Id: I5d8ab5c8f27c70f4af7051b24608ba0d1f2dd5cd