Set only global timestamp for non-exempt avalanche notifications

 During notification avalanche events, update the last notification posted timestamp
 for non-exmept notifications as the global timestamp only: this avoids attentuating
 exempt (eg. conversation) notifications that are posted after regular notifications
 during the avalanche events.

Flag: com.android.server.notification.polite_notifications_attn_update
Test: atest NotificationAttentionHelperTest
Bug: 357011208
Change-Id: I06edad789c8a85fe25e3385c7662d9f88d7eb0cf