Add support for notification scorers.

This CL adds an interface and classes for scoring notifications.
The NotificationManagerService initializes an array of scorers
specified as a resource. When a Notification is enqueued, the
getScore() method is called successively on the scorers, each
getting the Notification to be scored, and the score outputted
by the previous scorer. At present there is a single scorer
which prioritizes Notifications that mention the display name of
a starred contact.

To turn off the StarredContactNotificationScorer:
  adb shell settings put global contact_scorer_enabled 0

Change-Id: Ic16c80952e7c85bdde292ebb3f7900efb01f2e29