Support multiple connected Android devices in low bandwidth audio test

Previously it was assumed that exactly one device is connected.
Now adb will get an argument with the device ID taken from the runner
script's stdout.

BUG=webrtc:7229
TBR=kjellander@webrtc.org
NOTRY=true

Review-Url: https://codereview.webrtc.org/2783343003
Cr-Commit-Position: refs/heads/master@{#17580}

PRESUBMIT.py

@@ -470,7 +470,8 @@ def _RunPythonTests(input_api, output_api):
     return input_api.os_path.join(input_api.PresubmitLocalPath(), *args)
 
   test_directories = [
-      Join('webrtc', 'tools', 'py_event_log_analyzer')
+      Join('webrtc', 'tools', 'py_event_log_analyzer'),
+      Join('webrtc', 'audio', 'test', 'unittests'),
   ] + [
       root for root, _, files in os.walk(Join('tools-webrtc'))
       if any(f.endswith('_test.py') for f in files)