Reset initalization state of notif entries

When the notifications views are filtered out, reset their initialization
time, so now we can rely on the initialization time to determine whether
the notification was recently shown in the shade.

Fixes: 153885122
Test: atest NotificationRankingManagerTest
Test: manual
  1. post grouped notifications
  2. swipe down notification shade to view notifications
  3. toggle on/off DND and see notifications are not ungrouped when
they're unfiltered

Change-Id: Ib9b45cd13ead51d7df2484e91d159cd268799e9a