switch to new config access and handle read-only state, too

Change-Id: I499363ae59163ffcea4f7d3b0fe72289b88ffb79
Reviewed-on: https://gerrit.libreoffice.org/22261Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Andras Timar <andras.timar@collabora.com>