Build without KAuth and D-Bus on Android

Summary:
D-Bus isn't available on Android, and KAuth makes no sense either as the
concept of executing things with elevated access rights doesn't really
exist there.

This is part of an attempt of building Okular for Android without needing
the D-Bus stub.

Reviewers: dhaumann

Reviewed By: dhaumann

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18385