sal/osl/w32/security.cxx: don't use 8-bit string functions

We don't support Windows versions that don't have Unicode support.
In all currently-supported Windows versions, all FooA() functions
are actually wrappers for FooW() variants. We don't need to fallback
to A functions, and also don't need to do useless Unicode<->8-bit
conversions that also may result in data loss.

Change-Id: Ie21337c150ec0b9b4386c27d46f6596c14c4dd9f
Reviewed-on: https://gerrit.libreoffice.org/42281Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>