[platformsupport] Minor code style and class variables access changes

Summary:
Some of these variables have been declared protected but can apparently
also be private.

Other minor code style improvements.

Test Plan: Compiles, Wayland nested session runs.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22109