Set getDiffableValue() as synchronized in the Note model.

Fixes #1982
No need for AndroidAsync snapshot any longer.

AndroidAsync released an update, and Simperium 0.4.12 will use it.
Return a copy of the JSONObject in getDiffableValue().
Adding JSONUtilTest
Merge pull request #1990 from wordpress-mobile/issue/1982-simperium-concurrency-exception

Fix Simperium concurrency exception