Honor the sort and group keys for notification ranking.

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