bluez: Mark photos as initially up-to-date

The BlueZ backend only downloads contact photos if it determines its set
of photos is out of date (since BlueZ only lets it download _all_
photos, or _no_ photos). However, the variable controlling this could
never actually be set to true, so the backend always thought its set of
photos was outdated, and always re-downloaded them.
eds: Ignore non-local URIs for photos for security reasons

We can’t entirely trust the URIs provided to us in contacts, and they
could reference huge photos which we really don’t want to download (and
incur huge bandwidth bills). Since non-local URIs are really rare, it
seems reasonable to ignore them entirely, avoiding the security issue.

https://bugzilla.gnome.org/show_bug.cgi?id=697695
eds: Remove use of the AvatarCache

For several reasons:
 1) Now that only local URIs are permitted for URI-based photos, loading
    the photos should no longer be slow and need caching.
 2) Inline photos don’t need caching (and if they do, it should be in
    EDS, not folks — see bug #710826).
 3) The cached photos were never actually loaded again. How embarrassing.

https://bugzilla.gnome.org/show_bug.cgi?id=697695