[Android] Fix DeviceUtils.__str__ when no serial is provided.

tools/android/mempressure.py

Index: tools/android/mempressure.py
diff --git a/tools/android/mempressure.py b/tools/android/mempressure.py
index bd606f132235f70f1c4c762dff7d1e4f6a117e75..fa3daba3be6badd9a33ae7df108f6b9c9a4b6df0 100755
--- a/tools/android/mempressure.py
+++ b/tools/android/mempressure.py
@@ -15,8 +15,10 @@ BUILD_ANDROID_DIR = os.path.join(os.path.dirname(__file__),
                                  'build',
                                  'android')
 sys.path.append(BUILD_ANDROID_DIR)
+from pylib import android_commands
 from pylib import constants
 from pylib import flag_changer
+from pylib.device import device_errors
 from pylib.device import device_utils
 
 # Browser Constants
@@ -85,7 +87,12 @@ def main(argv):
   package = package_info.package
   activity = package_info.activity
 
-  device = device_utils.DeviceUtils(None)
+  devices = android_commands.GetAttachedDevices()
+  if not devices:
+    raise device_errors.NoDevicesError()
+  elif len(devices) > 1:
+    logging.warning('Multiple devices attached. Using %s.' % devices[0])
+  device = device_utils.DeviceUtils(devices[0])
 
   try:
     device.EnableRoot()