notification ranking infrastructure

Added an ordered list of notifications (n.b. a complete ordering).
Added a mechanism for ranking to be updated asynchronously
Added onNotificationRankingUpdate to NotificationListeners
Added an opaque order update object and a convenience comparator that
  uses it to sort notifications for listeners

Repurpose scorers to be ranking preprocessors.  The preprocessors will
perform heavy-weight validation of the notification object and memoize
the results to improve efficiency of the ranking comparator.

Current internal comparator implements status quo ordering, except
that notes with a valid contact sort to the top of their priority
bucket.

Change-Id: I7244c65944a9657df41fb313b3cb5a52e149709d