[PowerProfiler] Make sure correct version of pySerial is imported with monsoon profiler.

tools/telemetry/telemetry/core/platform/profiler/monsoon.py

 # 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.
 
 """Interface for a USB-connected Monsoon power meter.
 
 http://msoon.com/LabEquipment/PowerMonitor/
 Currently Unix-only. Relies on fcntl, /dev, and /tmp.
 """
 
 import collections
 import logging
 import os
 import select
 import struct
 import time
 
 from telemetry.core import util
 
-util.AddDirToPythonPath(util.GetTelemetryDir(), 'third_party', 'pyserial')
+util.InsertDirInPythonPath(0, util.GetTelemetryDir(), 'third_party', 'pyserial')
 import serial  # pylint: disable=F0401
 
 
 Power = collections.namedtuple('Power', ['amps', 'volts'])
 
 
 class Monsoon:
   """Provides a simple class to use the power meter.
 
   mon = monsoon.Monsoon()
@@ skipping 245 matching lines @@
         logging.error('exception from serial port')
         return None
       elif len(ready_r) > 0:
         flushed += 1
         self.ser.read(1)  # This may cause underlying buffering.
         self.ser.flush()  # Flush the underlying buffer too.
       else:
         break
     if flushed > 0:
       logging.debug('dropped >%d bytes', flushed)