Invoke test_droid with -s for device serial number

The commandline changed slightly, and this is now required.  See
b/27301538.

Bug: 27376986
Test: Invoking test_droid like this manually works.

Change-Id: I58f9a4eb10faeb918f6f2bf5b37aa4a8e49e5a47
diff --git a/builders/brillo_builders.py b/builders/brillo_builders.py
index 58f084e..7543606 100644
--- a/builders/brillo_builders.py
+++ b/builders/brillo_builders.py
@@ -198,7 +198,7 @@
       logging.info('Running tests against %s', serial)
       self.RunLunchCommand(
           ['external/autotest/site_utils/test_droid.py',
-           '--debug', serial, 'suite:brillo-smoke'],
+           '--debug', '-s', serial, 'suite:brillo-smoke'],
           cwd=self.BrilloRoot())