Fix a bug in handling for repeated any values.

If an API had a repeated any value marked as an additionalProperty, we'd fail
to decode, because we can't treat a JsonArray or a JsonValue with an
array_value as a list in Python.

We work around this in a painful way; in particular, there's a nasty inline
import because encoding.py needs to be split in two.

(reviewed internally)
diff --git a/default.pylintrc b/default.pylintrc
index bdea83a..1540a51 100644
--- a/default.pylintrc
+++ b/default.pylintrc
@@ -44,7 +44,9 @@
 
 [MESSAGES CONTROL]
 
+# TODO: remove cyclic-import.
 disable =
+    cyclic-import,
     fixme,
     import-error,
     locally-disabled,