Only exempt important conversations from notification avalanche

 Exempt only 'important conversations', not IMPORTANCE_HIGH notifications
 from being muted during the notification avalanche.

Flag: com.android.server.notification.polite_notifications_attn_update
Test: atest NotificationAttentionHelperTest
Bug: 341291320
Change-Id: I9020b6d1bdf98dca20f42a186e18a49af399027f