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