Fixup AndroidIntegrationTest broken by Distribution refactor.

This base test was using the old pre-DistributionLocator api.

Testing Done:
Failed with a missing symbol before, passes now:
```
$ ./pants test tests/python/pants_test/android/tasks:unpack_libraries_integration
```

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/80271642

Bugs closed: 1841, 2189

Reviewed at https://rbcommons.com/s/twitter/r/2811/

tests/python/pants_test/android/BUILD

@@ -28,6 +28,7 @@ python_tests(
   dependencies = [
     'src/python/pants/java/distribution:distribution',
     'tests/python/pants_test:int-test',
+    'tests/python/pants_test/subsystem:subsystem_utils',
   ],
 )
 

tests/python/pants_test/android/android_integration_test.py

@@ -7,8 +7,9 @@
 
 import os
 
-from pants.java.distribution.distribution import Distribution
+from pants.java.distribution.distribution import Distribution, DistributionLocator
 from pants_test.pants_run_integration_test import PantsRunIntegrationTest
+from pants_test.subsystem.subsystem_util import subsystem_instance
 
 
 class AndroidIntegrationTest(PantsRunIntegrationTest):
@@ -37,7 +38,8 @@ def requirements(cls, tools):
     else:
       return False
     try:
-      Distribution.cached(minimum_version=cls.JAVA_MIN, maximum_version=cls.JAVA_MAX)
+      with subsystem_instance(DistributionLocator) as locator:
+        locator.cached(minimum_version=cls.JAVA_MIN, maximum_version=cls.JAVA_MAX)
     except Distribution.Error:
       return False
     return True