Enable presubmit pylint in build/android.

build/android/host_heartbeat.py

 #!/usr/bin/env python
 #
 # Copyright (c) 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.
 
 """Sends a heart beat pulse to the currently online Android devices.
 This heart beat lets the devices know that they are connected to a host.
 """
 
-import os
 import sys
 import time
 
 from pylib import android_commands
 
 PULSE_PERIOD = 20
 
 def main():
   while True:
     try:
       devices = android_commands.GetAttachedDevices()
       for device in devices:
         android_cmd = android_commands.AndroidCommands(device)
         android_cmd.RunShellCommand('touch /sdcard/host_heartbeat')
     except:
       # Keep the heatbeat running bypassing all errors.
       pass
     time.sleep(PULSE_PERIOD)
 
 
 if __name__ == '__main__':
   sys.exit(main())