Forum activity backup and restore doesn't persist private reply state

If a forum that contains private replies is backed up and then restored the private replies will no longer be private in the newly restored activity.

Steps to replicate:

1. Log in as admin
2. Create a site with 2 users
3. Create a course with a forum
4. Enrol both users as students
5. Log in as user 1
6. Create a discussion in the forum
7. Log in as admin
8. View the discussion
9. Add a private reply to user 1's post
10. Backup the course (you don't need to include anything other than this forum activity)
11. Restore that backup into this course and have to add to the course (not override it)
12. Log in as user 2
13. View the original forum discussion
14. CONFIRM that you can't see the admin's private reply to user 1
15. View the restored forum discussion
16. CONFIRM that you now see the admin's private reply to user 1