Add option to specify ADB binary in test runner.

build/android/pylib/android_commands.py

Index: build/android/pylib/android_commands.py
diff --git a/build/android/pylib/android_commands.py b/build/android/pylib/android_commands.py
index 971fc4ee8f1a677e180f15229c5f6acfe1cbb0c4..8d7866dc9362bcc86b7d607f3e3923c4ee80900e 100644
--- a/build/android/pylib/android_commands.py
+++ b/build/android/pylib/android_commands.py
@@ -314,11 +314,7 @@ class AndroidCommands(object):
       device: If given, adb commands are only send to the device of this ID.
           Otherwise commands are sent to all attached devices.
     """
-    adb_dir = os.path.dirname(constants.GetAdbPath())
-    if adb_dir and adb_dir not in os.environ['PATH'].split(os.pathsep):
-      # Required by third_party/android_testrunner to call directly 'adb'.
-      os.environ['PATH'] += os.pathsep + adb_dir
-    self._adb = adb_interface.AdbInterface()
+    self._adb = adb_interface.AdbInterface(constants.GetAdbPath())
     if device:
       self._adb.SetTargetSerial(device)
     self._device = device