Make SvNumberFormatter public methods threadsafe

This is a requirement due to the recent threading work done
on calc formulagroup interpreter.

Change-Id: I5d567b4b957a4d86303f6a86b14d1cbfc730beda
Reviewed-on: https://gerrit.libreoffice.org/45085Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Dennis Francis <dennis.francis@collabora.co.uk>
(cherry picked from commit b1f829da)
Reviewed-on: https://gerrit.libreoffice.org/45328Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>