art_apex_test.py: Fix uses of APEX providers' field `_folder_cache`.

* Rename incorrect `folder_cache` uses to `_folder_cache`.
* Don't print field `_folder_cache` outside of APEX providers, as this
  field is supposed to be private; instead, use `List.print_list`.

Test: art/build/apex/runtests.sh
Change-Id: Ie22e0add501c4eb2b13b9cc4f3d8be3b3dc94c3d
1 file changed