De-flake notification avalanche tests

 Ensure that the notifications posted after an avalanche event will
  have a more recent 'when' timestamp.

Flag: com.android.server.notification.polite_notifications

Test: atest NotificationAttentionHelperTest
Bug: 371547369
Change-Id: I46ec71b69bc524b7d3d7fafb99615fd7bfbe5789