Android: Create a native view container

We need a way to embed native content in Qt, e.g. for WebView, multimedia, etc.

For QML, it would in theory be possible to redirect painting to a texture and use this as a node in the graph, but this would fail for WebViews containing video (since the video is a separate SurfaceView) and it would not solve the issue of texture-less multimedia. It would also require linking together the two event loops to propagate input events from Qt to Android, which can be a real pain.

It might be possible to create a place for the view in the QtLayout, so that it stacks on top of Qt's SurfaceView. I'm not sure how this will work for things like the video in WebView, which is already in a SurfaceView behind the WebView. It is possible that the stacking will be out of our hands in cases like that.