Set to old xml defaults IFF no specified policy for problem nor course.

@sarina @cpennington @Lyla-Fischer Can you test/review (as appropriate)?

* Need to test that xml backed lms behaves as expected when the policy file does and does not override rerandomize & showanswer at the course and problem levels (2 x 2)
* Need to test that importing an xml course into Studio where the same 2x2 are set/defaulted ends up behaving correctly
* Need to test that newly created problems in Studio
  * use the course overrides of those values if overridden at course in xml import
  * use the Studio defaults (never/completed) otherwise