Convert the `art_apex_test.py` script to Python 2.

This might help with build issues recently observed on the ART
Buildbot on the AOSP builders:
- aosp-builder-cc (https://ci.chromium.org/p/art/builders/ci/aosp-builder-cc);
- aosp-builder-cms (https://ci.chromium.org/p/art/builders/ci/aosp-builder-cms).

Test: m art-check-release-apex-gen-fakebin
Test: m art-check-debug-apex-gen-fakebin
Test: m art-check-testing-apex-gen-fakebin
Test: art/build/apex/runtests.sh
Test: Rely on ART Buildbot AOSP builders aosp-builder-cc and aosp-builder-cms
Bug: 139655167
Change-Id: I42574cbfd7c41c937483e3b81427cb9ce71f8925
2 files changed