Add Android notification backend

Summary:
There are probably still many features that aren't properly mapped to
Android, but at least the basics work:

• text, title and icon
• actions
• dismissing the notification is propagated
• taping the notification opens the corresponding app

This is also the first time we have Java code for Android in a KF5
framework, I tried to follow what Qt does for this.

Reviewers: apol

Reviewed By: apol

Subscribers: apol, nicolasfella, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17851