For me, I was facing this similar warning but then I updated my com.google.gms:google-services from 3.1.1 to 3.2.0 and the warning stopped appearing. Have shared the code below, hope this helps you out! 
buildscript { 
      repositories { 
              google() 
              jcenter() 
} 
dependencies { 
        classpath 'com.android.tools.build:gradle:3.1.0' 
// NOTE: Do not place your application dependencies here; they belong
// in the individual module build.gradle files 
    classpath 'com.google.gms:google-services:3.2.0' 
    } 
}