Download aar from https://github.com/zxzx74147/ExoPlayer/releases/tag/r2.11.8_transparency and import to you project.
Just change git url and the branch.
git clone https://github.com/zxzx74147/ExoPlayer.git
cd ExoPlayer
git checkout dev-v2-r2.11.8-transparency
Just set EXTENSION_RENDERER_MODE_PREFER.
DefaultRenderersFactory(this).setExtensionRendererMode(DefaultRenderersFactory.EXTENSION_RENDERER_MODE_PREFER);