Make notifications work on Android with API level < 23

Summary:
At API level < 23 we cannot set a custom icon but have to use a static one
from the Android resource system, so let's use the application icon for
that (without setting an icon Android isn't showing the notification).

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18274