Adjusts handling of multiple selected files.

Makes it so that the active element in the footer is always
shown.

fixes CNVS-15993

Test Plan:
  - Go to new files
  - Select multiple files to preview
  - The preview should work for only those
  - Close the preview
  - Make sure you have a bunch of files
  - Open the preview
  - Open the footer
  - Navigate through the files
  - The displayed file should always show up in the footer.

Change-Id: I47c24b4af8a9d27936c0aaebc760d9d901d605c6
Reviewed-on: https://gerrit.instructure.com/42318
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Clay Diffrient <cdiffrient@instructure.com>