After downloading index, remove apks no longer in the index.
It adds an extra 600ms on my Nexus 4 with ~2000 apks from the
F-Droid index. But I think it is the only way, as we really need
to iterate over every single installed apk, to see if it is still
wanted. The up side is that we can query for a large amount of
them, rather than quering individually for each apk.
NOTE: I haven't added a new status message yet, because we are
about to do a stable release. After the stable release, I'll
add a new status message to cover for this > half a second
(on my relatively fast device). This will probably be part of
an overhaul of the update process in general, including a
proper progress dialog.