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

tools/android/mempressure.py

 #!/usr/bin/env python
 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import collections
 import logging
 import optparse
 import os
 import sys
 
 BUILD_ANDROID_DIR = os.path.join(os.path.dirname(__file__),
                                  os.pardir,
                                  os.pardir,
                                  '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
 DEFAULT_BROWSER = 'chrome'
 
 # Action Constants
 ACTION_PACKAGE = 'org.chromium.base'
 ACTION_TRIM = {
     'moderate' : ACTION_PACKAGE + '.ACTION_TRIM_MEMORY_MODERATE',
     'critical' : ACTION_PACKAGE + '.ACTION_TRIM_MEMORY_RUNNING_CRITICAL',
@@ skipping 48 matching lines @@
     sys.exit(1)
 
   if not options.browser in constants.PACKAGE_INFO.keys():
     option_parser.error('Unknown browser option ' + options.browser)
 
   package_info = constants.PACKAGE_INFO[options.browser]
 
   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()
   except device_errors.CommandFailedError as e:
     # Try to change the flags and start the activity anyway.
     # TODO(jbudorick) Handle this exception appropriately after interface
     #                 conversions are finished.
     logging.error(str(e))
   flags = flag_changer.FlagChanger(device, package_info.cmdline_file)
   if ENABLE_TEST_INTENTS_FLAG not in flags.Get():
     flags.AddFlags([ENABLE_TEST_INTENTS_FLAG])
 
   device.StartActivity(intent.Intent(package=package, activity=activity,
                                      action=action))
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))