Prevent bell icon from disappearing on immediate followup notification.

The Ranking object of a notification update has lastAudiblylertedMs of
-1 if the notification did not buzz because a buzz happened within the
last 5s.  This causes the bell icon to disappear on a conversation
where a person posted twice within 5s. This is resolved by copying the
lastAudiblyAlertedMs from the NotificationEntry's existing Ranking when
setting a new one, if the new Ranking's value is -1.

Bug: 156887617
Test: Update an alerting notification within 5s of update which caused an alert; bell should stay even though buzz is skipped.
Change-Id: Ibb72ab27c88cfa57add781e2746fff0eee5de6be