Re-enable refreshing from library only on initial insertion of lib (#25347)

If the library content block is already in the course, then don't
refresh the children when we re-import it. This lets us address
TNL-7507 (Randomized Content Block Settings Lost in Course Import)
while still avoiding AA-310, where the IDs of the children for an
existing library_content block might be altered, losing student
user state.

When importing, we only copy the default values from content
in a library the first time that library_content block is created.
Future imports ignore what's in the library so as not to disrupt
course state. You can still update to the library via the Studio
UI for updating to the latest version of a library for this block.