Add Android notification channel support

Summary:
This is required for API level 28 or higher. This actually matches the
notfication events KNotification defines quite nicely, so those can be
individually configured now, like we are used to on other platforms.

Reviewers: apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18302