password: Fix changing the login keyring password

We are trying to update the login keyring password using uninitialized
memory as the old password from gis_driver_get_user_permissions() but
there's no password since we only set the first one below.

In fact, the gis-keyring API doesn't even need the "old" password to
be a parameter, we can just hardcode one which makes everything
simpler and works as fine for our purpose.

https://bugzilla.gnome.org/show_bug.cgi?id=738714