make notification preference buttons always screenreader accessible

fixes CNVS-14758

test plan
- ensure that buttons for all frequencies on the notification
 preference settings page are screenreader accessible

Change-Id: I4ae1651d8d9b7b702fce2b9226b5cb1f49e06403
Reviewed-on: https://gerrit.instructure.com/43186
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>