Fix string callbacks for special page registration

Currently registering a special page with a factory by passing the
callback string does not work because the is_string check comes
before the is_callback check. If we change the order of this if-
clauses we can also support callbacks like 'Factory::createPage'.

Change-Id: I751f97026ed8c580732c74d66b2804dd180ed16f
Merge "Fix string callbacks for special page registration"