Move onNotificationClear back to NEM

Due to an error in previous refactoring, onNotificationClear() was moved
to NotificationLogger even though onNotificationClear is not actually
logging - it's the actual call that informs system server that the user
has dismissed a notification. Therefore, we move onNotificationClear
back to NotificationEntryManager where it belongs.

Test: manual
Change-Id: I5c6ac847825e45a3c77114e910ad4db5059c738d