Making Unity player background as transparent in android app

I have an existing android application which is created based on Vuforia's Cloud reco project. The goal is to show 3D models once the target is recognized. I know there is a Unity project for Vuforia's Cloud reco but I don't want to go that route as I have already done god amount of work in my existing android app.

As a POC I created a simple android application and after some struggles. I'm able to bring in the Unity player inside the android app. I'm able to trigger a method on Unity C# from android which would download and load the model on the empty GameObject. Now here is my question

Currently the Unity player has a solid background (Main Camera from Unity). How do I make this background transparent so that I could show the 3D object on the surface/camera view in the android app?