Startcenter: use PAPER_SCREEN_4_3 as default paper for presentations

Remove static to able to construct items with different
paper info.

(cherry picked from commit 5ab1fb926e984780380665ae185646b603292a6e)

Change-Id: Ifba44675f6126ebfccf26f53f226214bed92352c
Reviewed-on: https://gerrit.libreoffice.org/7339
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>