Sobel operator implemented using renderscript

Hi.
I just implemented the sobel filter, that is used to approximate the image gradient, in a renderscript file.
This reduces the complexity of the application structure since a submodule is not needed anymore.

The proposed implementation implicitly offers a generic convolution for images. Therefore it can be used for other tasks later by using a different kernel (e.g. denoising...). I also added the Scharr filter kernels to be able to switch between sobel and scharr filter.

If you like it, feel free to merge :-)