Implement priority ordering in notifications.

Ongoings are the only notifications currently given higher
priority (and it's in an internal data structure, not a
public API, so fear not about abuse---this will be no worse
than on the phone where ongoings floated to the top).

The only thing left is to give privileged customers a way to
alter the priority of their notifications.

Bug: 3412807
Bug: 3146719
Change-Id: I9e738cc413982845cf4858faa8ccd0a7dbf3187c