Use valid auto-group notification trigger key for sparse groups

 When force grouping sparse groups, if the triggering notification is a summary,
  it will be removed and using it as the triggering notification will result in a NPE.
 Use another (first valid child) notification as the trigger in this case.

Flag: com.android.server.notification.notification_force_group_singletons
Test: atest GroupHelperTest

Bug: 366204575
Change-Id: I31e27a0130d58cd08a24f7db4f6fd44977c22abf