Temporarily change bgscan threshold in MultiSwitchBack

This makes the test more reliable by making it more
sensitive to the AP's change in signal level.  Also
use new method to reset background scan parameters
back to default after connection is made.

BUG=chromium-os:13938
TEST=Rerun test

Change-Id: I9abea39ee11177ffb876a75f6d1990270fbb7f4a

R=sleffler@chromium.org

Review URL: http://codereview.chromium.org/6805027
2 files changed