Unhide JSONObject.append()

The API is already referenced through documentation and is
recommended as a better alternative to JSONObject.accumulate().
Also, since it is already expected to be in use in some
applications, there doesn't seem to be a strong reason to keep
this hidden.

JSONObject tests already exercise this API.

Bug: 208933150
Test: m
Change-Id: If15a32a26e1d3c9ee9520fea64fbfb1c85be5711
2 files changed