Memory-optimizing Activity object not to keep duplicate data

Summary:
Activity object kept all the data that KActivities::Info keeps,

and an instance of KActivities::Info object. It is now just a thin
wrapper for the Info class.

Reviewers: davidedmundson, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2162