NoMan: Fix inconsistent notification comparator

Switch second phase of notification sorting to lexicographic
string comparison in order to enforce a consistent comparison.

Note that this slightly changes the logic: Within groups we
previously fell back on authoritative rank comparison when
one of the notificiations didn't have a sort key. Now we
just replace missing sort keys with " ".

Bug: 16626175
Change-Id: I91f4f4afeeb2c8edda6cd2668011a7367795086c