Fix last_devices to be quieter, and improve device affinity.

build/android/pylib/device/device_list.py

 # Copyright 2014 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.
 
 """A module to keep track of devices across builds."""
 
+import json
 import os
 
 LAST_DEVICES_FILENAME = '.last_devices'
-LAST_MISSING_DEVICES_FILENAME = '.last_missing'
 
 
-def GetPersistentDeviceList(file_name):
-  """Returns a list of devices.
+def ReadDeviceOfflineCountMap(file_name):
+  """Returns a dictionary of devices to the number of runs we have seen them
+  offline.  So 0 indicates that they were last seen online.
 
   Args:
     file_name: the file name containing a list of devices.
 
-  Returns: List of device serial numbers that were on the bot.
+  Returns: Dictionary mapping device ID to integer.
   """
   with open(file_name) as f:
-    return f.read().splitlines()
+    contents = f.read()
+  try:
+    return json.loads(contents)
+  except ValueError: # This might happen in old to new format transition.
+    return []
 
 
-def WritePersistentDeviceList(file_name, device_list):
+def WriteDeviceOfflineCountMap(file_name, device_map):
   path = os.path.dirname(file_name)
   if not os.path.exists(path):
     os.makedirs(path)
   with open(file_name, 'w') as f:
-    f.write('\n'.join(set(device_list)))
+    f.write(json.dumps(device_map))