New notification priority and related APIs.

This change introduces a few new bits of data on
Notification that will help the Notification Manager and
System UI route and display them more intelligently:

 -> priority: an integer in a predefined range that
    indicates the app's best guess as to the relative
    importance (to the user, right now) of that information

 -> kind: a tag (really, set of tags) indicating the general
    type of notification (realtime, asynchronous, etc)

 -> extras: a Bundle of additional key/value pairs
    associated with this notification (currently @hidden)

The notification manager takes these data into account when
assigning to each notification a score which is passed with
the notification on to the system UI, where it can be used to
affect presentation. For example:

  - Spammy apps (identified explicitly by the user or by
    some other means) will have their notifications scored
    very negatively by the notification manager, allowing
    the UI to suppress them
  - Notifications of higher score might be shown larger
    or in a different way
  - Very important notifications (indicated by a very high
    score) might interrupt the user during an otherwise
    important task (videochat, game, etc)

Implementation note: This replaces/extends the old internal
notion of "priority", which was mostly used to organize
ongoings and system notifications at the top of the panel.

Change-Id: Ie063dc75f198a68e2b5734a3aa0cacb5aba1ac39