Add a new usage event type for seen notification

On a notification becoming visible, NOTIFICATION_SEEN event is
dispatched. This will only bump up the app's bucket to WORKING_SET and
no higher. Same goes for sync adapters associated with a content
provider. Only move them to WORKING_SET.

Updated NotificationManagerService to report event to usagestats on
visibility changes.

Bug: 63527785
Test: runtest -x
frameworks/base/services/tests/servicestests/src/com/android/server/usage/AppStandbyControllerTests.java
atest NotificationManagerServiceTest
Change-Id: I5b132e1fc1f70e2126473b43b9b1979fbc523b85