[buteo-sync-plugins] Use "bluetooth" syncTarget when searching for and adding contacts.

Using a "bluetooth" syncTarget will allow the contacts backend to avoid
clobbering existing local contacts with data from a bluetooth-imported
contact.

With this patch:

1) When adding a contact, the contact is saved with a "bluetooth"
   syncTarget
2) When searching for contacts, we only return contacts with a
   "bluetooth" syncTarget. This ensures a sync operation does not
   modify or delete existing contacts that were not imported via
   bluetooth.