Miscellaneous non-functional enhancements

1. Addressed miscellaneous outstanding (non-functional) review comments from the previous refactoring and 3.7 update check ins:

a) better names for some APIDefinitions and added more comments
b) remove unneeded getCustomViews(DataViewType) method in UsersService (getCustomViews(CustomViewApiType) should be used instead)

2. Changed implementation of the uploadDocument(JSONDocument) (in DocumentsService)

Some Mambu Mobile users reported crashes with “out of memory” errors in the wrapper when running Gson.toJson(document) with large attachments. New custom implementation avoids unnecessary memory allocation in this method. Tested with large files.

3. Minor implementation change in the addAppKeyToJson() in RequestExecutorImpl.java (changed to appending JSON string instead of inserting into it).