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
diff --git a/build/apex/art_apex_test.py b/build/apex/art_apex_test.py
index 8058816..f670d57 100755
--- a/build/apex/art_apex_test.py
+++ b/build/apex/art_apex_test.py
@@ -157,7 +157,7 @@
 class HostApexProvider:
   def __init__(self, apex, tmpdir):
     self._tmpdir = tmpdir
-    self.folder_cache = {}
+    self._folder_cache = {}
     self._payload = os.path.join(self._tmpdir, 'apex_payload.zip')
     # Extract payload to tmpdir.
     apex_zip = zipfile.ZipFile(apex)
@@ -176,12 +176,12 @@
     return apex_map[name] if name in apex_map else None
 
   def read_dir(self, apex_dir):
-    if apex_dir in self.folder_cache:
-      return self.folder_cache[apex_dir]
-    if not self.folder_cache:
+    if apex_dir in self._folder_cache:
+      return self._folder_cache[apex_dir]
+    if not self._folder_cache:
       self.parse_zip()
-    if apex_dir in self.folder_cache:
-      return self.folder_cache[apex_dir]
+    if apex_dir in self._folder_cache:
+      return self._folder_cache[apex_dir]
     return {}
 
   def parse_zip(self):
@@ -205,9 +205,9 @@
         apex_dir, base = os.path.split(path)
         # TODO: If directories are stored, base will be empty.
 
-        if apex_dir not in self.folder_cache:
-          self.folder_cache[apex_dir] = {}
-        dir_map = self.folder_cache[apex_dir]
+        if apex_dir not in self._folder_cache:
+          self._folder_cache[apex_dir] = {}
+        dir_map = self._folder_cache[apex_dir]
         if base not in dir_map:
           if is_zipinfo:
             bits = (zipinfo.external_attr >> 16) & 0xFFFF
@@ -950,7 +950,7 @@
 
 
 class List:
-  def __init__(self, provider, print_size):
+  def __init__(self, provider, print_size=False):
     self._provider = provider
     self._print_size = print_size
 
@@ -981,7 +981,7 @@
 
 
 class Tree:
-  def __init__(self, provider, title, print_size):
+  def __init__(self, provider, title, print_size=False):
     print('%s' % title)
     self._provider = provider
     self._has_next_list = []
@@ -1100,7 +1100,7 @@
       test_args.bitness = '64'
     else:
       logging.error('  Could not detect bitness, neither lib nor lib64 contained.')
-      print('%s' % apex_provider.folder_cache)
+      List(apex_provider).print_list()
       return 1
 
   if test_args.bitness == '32':