This has no dependencies other than GSON, and putting it in its own
package simplifies the top-level server package, especially if we add
more classes later.
Not all of this class is actually JSON-related, namely the TEXT value.
But most of it is, and this was easier than factoring out the
GsonBuilder logic into a separate class.
Change-Id: I6c9e49a52d8e27360f3dba2356db7a8d24beb6fe
87bfe687af