Adapt to LibKGAPI setFields() changes.

Summary:
In the next release, LibKGAPI has changed the way it sets fields in FileFetchJob and in general allows setting response fields for all Jobs (see D20886 Add fields standard query parameter to Job and use it in FileFetchJob).

This patch adjusts KIO Gdrive to this change and limits the response in AboutFetchJobs.

Reviewers: dvratil, elvisangelaccio

Subscribers: LibKGAPI, KIO GDrive

Tags: KIO GDrive, LibKGAPI

Differential Revision: https://phabricator.kde.org/D20888