Check sparse group child notifications for trigger section

  Do not force-group sparse groups if the child notifications are not groupable. In some groups, the summary may match a valid section but the child notifications do not: ie. conversations where the summary is not MessageMessagingStyle.

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

Test: atest GroupHelperTest
Bug: 377788156
Change-Id: I92a33a3a47ebbf46556e080beb356a1d5962e1b6