Implement lock-screen visibility control on Android

Summary:
This is only the backend part, lacking a proper frontend API this is
using a custom hint for now. Android knows three levels for this:

• public: visible completely on the lock screen
• private: lock screen only shows that $app has notifications
• secret: nothing shown on the lock screen at all

Reviewers: nicolasfella

Reviewed By: nicolasfella

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29358