how to put Activity(rootview, something else) to SurfaceTexture

Question

I want to make one Android application part of another application (something like the browser in a unity 3d game) and for this the first one provides the second SurfaceTextures(can use only this way). Are there any code examples how can I put(render) normal android activity(maybe rootview, idk) into this SurfaceTexture? Maybe something like using https://github.com/google/grafika/blob/master/app/src/main/java/com/android/grafika/PlayMovieSurfaceActivity.java ?