build.gradle

buildscript {   
ext.kotlin_version = '1.2.21'
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath "your.dependency:module:x.y.z"
    }
}
import your.dependecy.Dependecy

dependency {
  implementation Dependency.Rx.rxJava
  implementation Dependency.Rx.rxKotlin
}

Dependecy.java

package your.dependency

interface Dependecy {
  interface Rx {
    String rxJava = "io.reactivex.rxjava2:rxjava:${rxJavaVersion}";
    String rxKotlin = "io.reactivex.rxjava2:rxkotlin:${rxKotlinVersion}";
  }
}