Allow import of native XBlocks in draft

[TIM-443](https://edx-wiki.atlassian.net/browse/TIM-443): XBlock set to Private does not import

There were three underlying issues:
1) The `ImportSystem` wasn't providing the XBlock with an in-memory key-value store, so reads/writes to XBlock fields during import would raise exceptions.
2) The `ImportSystem` wasn't mixing the XModule compatibility class into "pure" XBlocks, so `module.location` was not available.
3) The XBlock's key value store was not being re-keyed after changing the location, so fields modified during import were not being persisted.  (Same fix as #3225)

@cahrens @ormsbee