Android P will be supporting multicamera mode so that developers/users can access both physical cameras. Depth calculation from monochrome is not a problem because most algorithms use the stereo pictures after converting them to gtay scale. In current versions the difficulty is only getting access to both physical cameras. I am too waiting for this Android release.
This one is written in OpenCV python using grayscale image as input.
https://docs.opencv.org/3.0-beta/doc/py_tutorials/py_calib3d/py_depthmap/py_depthmap.html
