Use exposed DBus methods to switch activities in CLI

Summary:
Since https://phabricator.kde.org/D22381 has landed kactivitymanagerd
now exposes methods to switch to the previous/next activity via DBus.

We can call them from kactivities-cli and remove some duplicated code.

Reviewers: ivan

Reviewed By: ivan

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24401