Fix the output of art_apex_test.py's `--list` option.

Test: Run "art/build/apex/runtests.sh -l" and check the output
Change-Id: I3c743f4ceb39f6491d7384cef1a794ae8039ac6e
diff --git a/build/apex/art_apex_test.py b/build/apex/art_apex_test.py
index dc95f93..31358a0 100755
--- a/build/apex/art_apex_test.py
+++ b/build/apex/art_apex_test.py
@@ -615,10 +615,13 @@
     if '..' in apex_map:
       del apex_map['..']
     for (_, val) in sorted(apex_map.items()):
-      self._path = os.path.join(self._path, val.name)
-      print(self._path)
+      val_path = os.path.join(self._path, val.name)
+      print(val_path)
       if val.is_dir:
+        saved_dir = self._path
+        self._path = val_path
         self.print_list()
+        self._path = saved_dir
 
 
 class Tree: