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

build/android/pylib/device/device_utils_test.py

Index: build/android/pylib/device/device_utils_test.py
diff --git a/build/android/pylib/device/device_utils_test.py b/build/android/pylib/device/device_utils_test.py
index b6a99d28d4bb7fb85a0a0b00c37283a1063d2d26..a6ac8755101f4d3060c0f98d70cb1e52b0b6a795 100755
--- a/build/android/pylib/device/device_utils_test.py
+++ b/build/android/pylib/device/device_utils_test.py
@@ -1484,6 +1484,11 @@ class DeviceUtilsStrTest(DeviceUtilsOldImplTest):
     with self.assertNoAdbCalls():
       self.assertEqual('0123456789abcdef', str(self.device))
 
+  def testStr_noSerial(self):
+    self.device = device_utils.DeviceUtils(None)
+    with self.assertCalls('adb  get-serialno', '0123456789abcdef'):
+      self.assertEqual('0123456789abcdef', str(self.device))
+
 
 if __name__ == '__main__':
   logging.getLogger().setLevel(logging.DEBUG)