srgb - focus changes on next/previous buttons

fixes CNVS-11222

test plan:
  - go to srgb with screenreader on
  - click next assignment until there are no more
    - you should refocus on previous assignment and it should say so
  - click previous assignment until there are no more
    - you should refocus on next assignment and it should say so
  - do the same thing with next/prev students

Change-Id: Ie0e8a5d2a01c3988ae1d2edc6e089602fba7b719
Reviewed-on: https://gerrit.instructure.com/31534
QA-Review: Mike Nomitch <mnomitch@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>