Don't load KDE platform module in kactivitymanagerd

Summary:
kactivitymanagerd doesn't do anything that requires it to be
aware of KDE GUI settings (colors, style, standard keys, etc).

This makes it start a fair bit faster.

Reviewers: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D9193