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

build/android/pylib/device/device_utils.py

Index: build/android/pylib/device/device_utils.py
diff --git a/build/android/pylib/device/device_utils.py b/build/android/pylib/device/device_utils.py
index 997405a07d6e570d425ceeacbfe382a7c238becb..8a8de22519f3d69bc93681657494ed076eaeba05 100644
--- a/build/android/pylib/device/device_utils.py
+++ b/build/android/pylib/device/device_utils.py
@@ -790,7 +790,12 @@ class DeviceUtils(object):
 
   def __str__(self):
     """Returns the device serial."""
-    return self.old_interface.GetDevice()
+    s = self.old_interface.GetDevice()
+    if not s:
+      s = self.old_interface.Adb().GetSerialNumber()
+      if s == 'unknown':
+        raise device_errors.NoDevicesError()
+    return s
 
   @staticmethod
   def parallel(devices=None, async=False):