In historical Fabric/Crashlytics for Android documentation and various online examples there is mention of the Fabric 'apiSecret' that should be included in your fabric.properties file. However, in the latest official documentation for setup with Gradle, there is no mention of it: https://fabric.io/kits/android/crashlytics/install. It also appears that the Gradle setup is no longer using fabric.properties and instead using the apiKey defined in the AndroidManifest.xml.
What is the recommended way to handle the 'apiSecret' and 'apiKey' in Android Gradle projects?
Currently Google is making the transition from fabric/crashlytics towards firebase/crashlytics. I'd recommend you look into this guide if you're currently implementing crashlytics
https://firebase.google.com/docs/crashlytics/
If going this route, you have to download a .json file from firebase after creating an app project there and include it in your app. No key needed.