Set FLAG_SILENT to group summaries with all childen bundled

 If a group summary is posted and all its children were bundled during enqueue (not yet regrouped):
  - set FLAG_SILENT so that it does not alert, being an empty group

Flag: EXEMPT bugfix

Test: atest NotificationManagerServiceTest
Bug: 420894685
Change-Id: I6f623f68b063fe4591d465e81a12bf8f2207acf4