[qtcontacts-sqlite] Preserve the existing ID of an exported contact

If a contact is exported to a sync adaptor, then a remote modification
subsequently causes it to acquire a constituent from that sync adaptor,
ensure that it continues to be exported to that sync target with the
ID of the local constituent it originated from.

This introduces the possibility of having non-local contacts marked
as incidental, which has the effect of preventing them from being used
as the base of a partial aggregate.