Assign: Allow changing marking workflow in the app

When grading an assign with marking workflow enabled, in LMS the teacher can change the marking workflow but the app doesn't allow it, it only displays the current one and cannot be changed. The WebService mod_assign_submit_grading_form supports sending the new workflow state, so this could probably be supported in the app too.

This can be reproduced in the course named MDL-72047 in our master testing site.