Restore deleted conversation channel on notification posted

 Restore a deleted conversation channel when posting a conversation
  notification. Previously, the conversation channel was restored/created
  when opening the notification guts (long-press on notification). This caused some notifications be posted with sound even though the user chose to silence the conversation via guts because the channel was deleted on reposting: b/280235550.

Test: atest com.android.server.notification.NotificationManagerServiceTest#testRestoreConversationChannel_deleted
Test: atest PreferencesHelperTest
Bug: 304016442

Change-Id: I8be8e280e9a87278635228c7a9dd5baa2e29eb26