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

build/apex/Android.bp

@@ -375,10 +375,10 @@ python_binary_host {
     main: "art_apex_test.py",
     version: {
         py2: {
-            enabled: false,
+            enabled: true,
         },
         py3: {
-            enabled: true,
+            enabled: false,
         },
     },
 }

build/apex/art_apex_test.py

@@ -1,4 +1,5 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
 
 # Copyright (C) 2019 The Android Open Source Project
 #