Fix empty HUN group header when async group header is enabled

Fixes the issue that when posting a high-priority
notification group with the alerting behavior:
GROUP_ALERT_SUMMARY, the heads-up notification group has
empty group header.

Fix: 331218195
Flag: ACONFIG notification_async_group_header_inflation STAGING
Test: PlatformScenarioTests
TapChildOfHeadsUpSummary#tapChildOfHeadsUpSummary

Change-Id: If55c9f801ab742ae17a3409f0ea202e288b1356a