optimize progress updating on content imports progress bar

Previously, we were adding a new listener to the completion change value
on every render, which meant that our # of listeners was growing
exponentially.

test plan:
 * Do a course copy.
 * On the progress screen, make sure the progress bar still works.

Change-Id: I4af568f66123dd053beb76626645f07d908cfdb3
Reviewed-on: https://gerrit.instructure.com/40346
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>