If no device specified, get the first device found

This makes adb_reverse_forwarder.py a little more convenient
to use.

BUG=429859

Review URL: https://codereview.chromium.org/684383004

Cr-Commit-Position: refs/heads/master@{#302548}

build/android/adb_reverse_forwarder.py

@@ -15,6 +15,7 @@ import optparse
 import sys
 import time
 
+from pylib import android_commands
 from pylib import constants, forwarder
 from pylib.device import device_utils
 from pylib.utils import run_tests_helper
@@ -50,7 +51,19 @@ def main(argv):
     parser.error('Bad port number')
     sys.exit(1)
 
-  device = device_utils.DeviceUtils(options.device)
+  devices = android_commands.GetAttachedDevices()
+
+  if options.device:
+    if options.device not in devices:
+      raise Exception('Error: %s not in attached devices %s' % (options.device,
+                      ','.join(devices)))
+    devices = [options.device]
+  else:
+    if not devices:
+      raise Exception('Error: no connected devices')
+    print("No device specified. Defaulting to " + devices[0])
+
+  device = device_utils.DeviceUtils(devices[0])
   constants.SetBuildType(options.build_type)
   try:
     forwarder.Forwarder.Map(port_pairs, device)