cp#100030: Use internal API to fill dxf styles during xlsx import.

It's faster, and it actually prevents a bug where styles fail to get
set when the document contains at least one protected sheet.

Change-Id: I9a5d7250ca44dd46364ab62f2e0130e967427c66
(cherry picked from commit d12c0e5d36cc106cb4f9a27dba2f29c10b5d6f93)
Reviewed-on: https://gerrit.libreoffice.org/7982
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>