Notification API hardening: add statsd logs for forced auto-grouping events

 Add NotificationEvents:
  - NOTIFICATION_FORCE_GROUP: autogrouped child notification without summary
  - NOTIFICATION_FORCE_GROUP_SUMMARY: autogrouped summary notification without children
 Sparse/singleton groups summary cancelation is tracked by REASON_GROUP_OPTIMIZATION.

 Also added EventLog NOTIFICATION_SUMMARY_CONVERTED.

Flag: android.service.notification.notification_force_grouping
Flag: com.android.server.notification.notification_force_group_singletons
Flag: android.service.notification.notification_silent_flag

Test: manual via statsd_testdrive

Bug: 336488844
Change-Id: I83704994166f36c44742aeacfa7a567594c13afb