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.
Expose a new API for listeners to get the group key for the notificaiton.
Bug: 15190903
Change-Id: I324ba0c394affdabb3588ca2ebafa7cf0acad2af
1031c97485