[Notif] Allow locking importance on notification

Currently locking  only works on channel - this CL allows doing
so on the overarching notification too. Added locking field in
appropriate places in record and surfaced it to other bits via
the RankingHelper.

Test: Visually, reproduced organically (+ deleting /data/ files)
Fixes: 77775657
Change-Id: Ie46093921dd6c1ae3533ded7b87faaa475a631e4