Overview Desks: Display close button for switch access

Make the close desk button visible and navigable for switch access
users.

Switch access users are unable to execute keyboard commands through
their physical switch devices, rendering them unable to close virtual
spaces without the close buttons being visible.

Test: manual
Change-Id: I8624783a586424307971f0a27ba9c80456fa788b
Fixed: 997011
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391594


Commit-Queue: Richard Chui <richui@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805597}