Install phosh-portals.conf in /usr/share instead of /etc

Follow-up for !1301 (merged).

Since xdg-desktop-portal 1.17.1, desktop environments can/should install their default foo-portals.conf into /usr/share/xdg-desktop-portal, reserving /etc/ for sysadmin configuration (sometimes called "hermetic /usr").

1.17.0 was a development prerelease and was superseded by 1.17.1, so I think it's fine to ignore 1.17.0 and only support 1.17.1. The change should be as simple as this (entirely untested, I don't use Phosh):

-install_data('phosh-portals.conf', install_dir : join_paths(sysconfdir, 'xdg-desktop-portal'))
+install_data('phosh-portals.conf', install_dir : join_paths(datadir, 'xdg-desktop-portal'))

/cc @guidog