mTLS for Android

I implemented mTLS for android app following this

But when i upload my release build to PlayStore, i met this error:
Security Alert: Your app contains embedded private keys or keystore files

I know because of this
For demonstration purposes, the certificate in this example is stored in app/src/main/res/raw/cert.pemand the private key is stored inapp/src/main/res/raw/key.pem . You may also store these files in other secure manners.

So I want to ask: To avoid that PlayStore error, what can be other secure manners?