autotest: Add WiFi signal level property to WiFi client

TEST=A test that consumes this property is able to do so effectively on
daisy.
BUG=chromium:279395

Change-Id: I916492df83e61cedd0fbbaf273c94970bf03e2dd
Reviewed-on: https://chromium-review.googlesource.com/66713
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
2 files changed