Android: QScreen grabWindow() returns null pixmap

When refactoring the platform windows for child window support, raster windows were changed to have their own surface instead of using the screen's surface, and to use OpenGL underneath. Due to this, the previous implementation for grabWindow() is no longer suitable, and we have regressed to not implement the method on Android platform screen -> returning a null QPixmap.

Reading the git history for the method, it has only ever worked for raster (=widget) windows, and OpenGL ones have always returned a null QPixmap.