Condense NotifSection + index into a wrapper class

Renames NotifSection -> NotifSectioner

Creates a new wrapper class, NotifSection, that's the combination of a
sectioner and its assigned index.

Removes a bunch of places where we were using two separate members to
store both the sectioner and its index.

Test: atest
Change-Id: I522b68aa1521172e42b7c934ca8280f41fa3c2da