bnc#885548: Set the date time mode to non-fixed after the import is done.

So that the new revisions will correctly record time stamps.

Change-Id: I7d9b26bed04a95ce2652224faa0f214562818970
(cherry picked from commit 2a6843cb6c44305ac62c9cd1098a3eec4abaeb7e)
Reviewed-on: https://gerrit.libreoffice.org/10829
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>