import mock from third_party explicitly

Stop relying on third_party/ leaking into sys.path all the time.

BUG=b:187794934
TEST=CQ passes

Change-Id: If233859063e6d2a1f2fdaa851febcbe8f536896a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2896053
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/api/controller/android_unittest.py b/api/controller/android_unittest.py
index 7d94d4b..9c248bb 100644
--- a/api/controller/android_unittest.py
+++ b/api/controller/android_unittest.py
@@ -7,19 +7,18 @@
 
 from __future__ import print_function
 
-import mock
-
 from chromite.api import api_config
 from chromite.api.controller import android
 from chromite.api.gen.chromite.api import android_pb2
 from chromite.api.gen.chromiumos import common_pb2
+from chromite.lib import build_target_lib
 from chromite.lib import constants
 from chromite.lib import cros_build_lib
 from chromite.lib import cros_test_lib
 from chromite.lib import osutils
-from chromite.lib import build_target_lib
 from chromite.service import android as service_android
 from chromite.service import packages
+from chromite.third_party import mock
 
 
 class GetLatestBuildTest(cros_test_lib.MockTestCase, api_config.ApiConfigMixin):