When transferring multiple files via Bluetooth, we quickly receive
one transfer request after another, so this causes a timing issue
where the engine may quit immediately after the first request and
is in the process of closing down when the next request is received,
and thus cannot be started again for the next request.

Review fixes

Allow MediaItem::ResourceName to be passed to sendNotification() when
MediaItem::Url is empty, as it will be if an uploader is sending a
non-file resource such as a vCard.