[a11y] Announce network connection status in tray.

When navigating through the network list in quick settings, announce
the connection status of the highlighted network when available.

Bug: 1006005
Change-Id: Ib2a640af72742817118dea76e2b210e12180bff0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879435
Commit-Queue: Jon Mann <jonmann@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710457}