Ensure cancelation order for groups in clearAll

Group summaries will be canceled before their children, to
prevent situations where a child cancelation causes a group summary
update which interferes with the cancelation of the summary from
the clear all action.

Additionally this fixes a bug where child notifications that
are priority conversations where not being canceled during clear all.

Test: atest NotificationManagerServiceTest, atest NotificationManagerTest
Fixes: 148387460
Change-Id: I21d34444e5097ec1efc6e06654ced9f894399bf4