[A11y] Fix reading of bluetooth items in tray menu.

This CL fixes issue with bluetooth device list items in tray
menu losing focus and reading out device name repeatedly. This
was caused because all item views in the list are recreated
every time the device list updates. Fixed this by re-using and
re-ordering existing view items instead of deleting and
re-creating every time.

Fixed: 1111166, 1007371
Change-Id: I38c31e80fafec0bc80dd62bf37ff1b926ccf0014
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2465518
Commit-Queue: Azeem Arshad <azeemarshad@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816655}