Fix repeat HUNs after autogrouping

Set FLAG_SILENT from:
addAutogroupKeyLocked
updateAutobundledSummaryLocked

Note that we cannot call fixSilentGroup from these places because
fixSilentGroup checks for GROUP_KEY_SILENT to fix data from
androidx apps. We want to set FLAG_SILENT regardless of mGroupKey

Bug: 396573298
Test: send delayed HUNs that are each marked as group summary,
      enabled force_grouping flag, see that HUNs do not repeat
Flag: android.service.notification.notification_force_grouping
Change-Id: I22c6898f78f190e9f8cb3be8c2b626602c2e1f73