fdo#79269: fix ODF import of style:footer-first

The implementation of SwXStyle's FirstIsShared property is busted, and
that causes xmloff to write the footer-first content into the master
footer.

This is a re-implementation of 0f21f932081471b2a5eda820fa1a194fbf3ab85c
because unostyle.cxx is very different on master...

Change-Id: I485d8089c1fb5ba3ca369fbfe8f3115bf700ba13
Reviewed-on: https://gerrit.libreoffice.org/11722
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>