update enrollment activity while window is open

fixes ALYT-32

test plan:
  * open a course page as a student
  * check the student's enrollment's total activity time
  * leave the page open and in the foreground for at least three minutes
  * verify that the student's activity time is updated

Change-Id: I0448ea48dc6739ae1199996d0a4b88b030f43856
Reviewed-on: https://gerrit.instructure.com/35553
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>