a11y: Conversations hidden filter labels now contain the value selected

Fixes CNVS-17030

Test Plan:
- Using a screen reader visit conversations.
- Navigate to the type and course filters
- Verify that screen readers read the selected item and the filter label,
  without you having to open the drop-down.
- Change the selected value and close the dropdown.
- Verify the label is updated with the new selection.

Change-Id: I27611cc5114a51bdf351ec76306337e0cb606825
Reviewed-on: https://gerrit.instructure.com/45051
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
QA-Review: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>