Sort notifications naturally, then move group childen to be next their proxy.
The group proxy is the summary, or if no summary exists, the lowest-ranked
member of the group is chosen as the proxy.
Notifications with a sortKey but no group and placed into a synthetic
group that consists of all notifications from that package and user in
the same priority bucket that also have sortKeys.
Bug: 15190903
Change-Id: I377ed65b9592079446da4a4189f5ab49d28630ec
4a24fd3c7b