[libcontacts] Use isOnline flag to select FilterOnline contacts

Note that the presence of this flag in the metadata means we should not
need to query for these contacts explicitly; we should instead extract
the subset of online contacts from the metadata contact set, and
insert those directly into the models.
Merge pull request #11 from matthewvogt/online-flag

[libcontacts] Use isOnline flag to select FilterOnline contacts