Fix audience check in Notifications

- before:  would try to determine the user ID of the current user and use
  that to figure out which other person with which the post was shared.
  This only worked on the G+ Profile page, though, not within the
  Notifications panel (which seems to have no clear way to determine who the
  current user is).

- after:  just show both user names of the two people with whom a private
  post was shared.  The string is a little longer, but it works in all
  cases.