Add option to specify ADB binary in test runner.

build/android/pylib/device/adb_wrapper.py

Index: build/android/pylib/device/adb_wrapper.py
diff --git a/build/android/pylib/device/adb_wrapper.py b/build/android/pylib/device/adb_wrapper.py
index e7a84185d8f43b81a3c641ed0669622e4c4d6025..cb8cf7d627841247c6cbefd47b2bde9a9d56c0b4 100644
--- a/build/android/pylib/device/adb_wrapper.py
+++ b/build/android/pylib/device/adb_wrapper.py
@@ -12,6 +12,7 @@ import errno
 import os
 
 from pylib import cmd_helper
+from pylib import constants
 from pylib.device import decorators
 from pylib.device import device_errors
 from pylib.utils import timeout_retry
@@ -49,7 +50,7 @@ class AdbWrapper(object):
   @classmethod
   @decorators.WithTimeoutAndRetries
   def _RunAdbCmd(cls, arg_list, timeout=None, retries=None, check_error=True):
-    cmd = ['adb'] + arg_list
+    cmd = [constants.GetAdbPath()] + arg_list
     exit_code, output = cmd_helper.GetCmdStatusAndOutputWithTimeout(
       cmd, timeout_retry.CurrentTimeoutThread().GetRemainingTime())
     if exit_code != 0: