Add Android platform calendar plugin

This is essentially just moving code that has been working in Itinerary for years. This now became movable without dragging along a whole bunch of template machinery for the type-safe JNI use thanks to Qt providing large parts of this itself meanwhile.

This only covers events, as Android has no native support for tasks or journals. Extensions for those exist though, and could eventually also supported here.

What this doesn't cover is requesting runtime calendar permissions, that's still for the application to manage (static permissions are handled by just linking against KCalendarCore).