Store them in one place

store_in_one_place.java

//build.gradle
android{
  ...
  productFlavors{
    staging{
      //use in java code
      buildConfigField "String", "SERVICE_THAT_I_NEED", "s3cr3t_k3y_f0r_hackz0rz_staging"
      buildConfigField "String", "MY_BACKEND_URL", "https://www.staging-example.com/api"
      //use in xml, like AndroidManifest.xml
      resValue "string", "google_maps_key", "5435651423234"
    }
    
    production{
      //use in java code
      buildConfigField "String", "SERVICE_THAT_I_NEED", "s3cr3t_k3y_f0r_hackz0rz_production"
      buildConfigField "String", "MY_BACKEND_URL", "https://www.production-example.com/api"
      //use in xml, like AndroidManifest.xml
      resValue "string", "google_maps_key", "5435651423234"
    }
  }
}