I am trying to build a Cardboard android application that shows 2 camera view side by side. [Just like the camera view works for VRCinema Android app.]
Basically you need to draw the camera into an OpenGL texture and show the texture.
I wrote two versions of this here, feel free to modify the code and add pull requests: https://github.com/Sveder/CardboardPassthrough
I updated @Mikle code to work with the new GoogleVR SDK:
https://github.com/SalahEddin/DJI-Vision/commit/45fe28302d96ef4b1cd07472997d9feb1da5d009