Privatize variables in InputDeviceHandler

Summary:
Some members were declared protected. Better style is to have them private
with public or protected getters and setters.

This also removes the unnecessary m_input variable.

Test Plan: Builds and runs.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15519