convert specialised SvStream::operator>> methods to ReadXXX methods

as preparation for converting the SvStream::operator>> methods on
primitive types

Change-Id: I62f134bced15c687d6e0d46924f56e8d1c3d95b9
Reviewed-on: https://gerrit.libreoffice.org/7798
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>