Fix notification grouping and section bug

Update HighPriorityProvider to check whether a group summary has a child
with high priority - if so, consider the summary high priority. This was
implemented for the new pipelne but not for the old pipeline grouping
structure. This retains the functionality pre-R where a group of low
priority and high priority notifications gets sorted into the "Alerting"
section instead of "Silent".

In NVHM, guard against summaries appearing after its children in the
sorted list of notifications returned by NEM.getVisibleNotifications.
Previously, summaries appearing after its children would remove the
children from the final notification shade list.

Test: atest SystemUITests
Fixes: 156407436
Change-Id: Id023047360249370e94085d62705d2977b03056e