Demonstration of how to extract dependencies from your build.gradle file.

build.gradle

// Top-level build file where you can add configuration options common to all sub-projects/modules.
// Load dependencies


buildscript {

    ext.kotlin_version = "1.3.0"
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.2.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:${kotlin_version}"

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }


}

allprojects {
    repositories {
        google()
        jcenter()
    }

}

task clean(type: Delete) {
    delete rootProject.buildDir
}
apply from: 'gradleScripts/dependencies.gradle'

gradleScripts_dependencies.gradle

ext {
    //Version
    supportLibrary = '28.0.0'
    rxjava = "2.2.2"
    rxandroid = "2.1.0"
    dagger = "2.17"
    mockwebserver = "3.11.0"
    gson = "2.8.5"
    mockito = "2.22.0"
    firestore = "17.1.0"
    firebaseui = "4.1.0"
    retrofit = "2.4.0"
    constraintlayout = "1.1.3"
    archextensions = "1.1.1"

    constraintlayout = [
            //api
        lib : "com.android.support.constraint:constraint-layout:$constraintlayout"
    ]

    archextensions = [
            lib: "android.arch.lifecycle:extensions:$archextensions"
    ]

    //Support Libraries dependencies
    supportDependencies = [
            design           : "com.android.support:design:${supportLibrary}",
            recyclerView     : "com.android.support:recyclerview-v7:${supportLibrary}",
            cardView         : "com.android.support:cardview-v7:${supportLibrary}",
            appCompat        : "com.android.support:appcompat-v7:${supportLibrary}",
            supportAnnotation: "com.android.support:support-annotations:${supportLibrary}",
    ]

    rxjava = [
            lib    : "io.reactivex.rxjava2:rxjava:${rxjava}",
            android: "io.reactivex.rxjava2:rxandroid:${rxandroid}"
    ]

    dagger = [
            // Dagger
            lib                : "com.google.dagger:dagger:${dagger}",
            kapt               : "com.google.dagger:dagger-compiler:${dagger}",
            android            : "com.google.dagger:dagger-android:${dagger}",
            support            : "com.google.dagger:dagger-android-support:${dagger}",
            annotationProcessor: "com.google.dagger:dagger-android-processor:${dagger}",
    ]

    mockwebserver = [
            lib: "com.squareup.okhttp3:mockwebserver:$mockwebserver"
    ]

    gson = [
            lib: "com.google.code.gson:gson:$gson"
    ]

    mockito = [
            lib    : "org.mockito:mockito-core:$mockito",
            android: "org.mockito:mockito-android:$mockito"
    ]

}

Create a seprate file, for instance, a dependencies.gradle under the folder gradleScripts in the root of your project.
Then, in the project level build.gradle:
apply from: 'gradleScripts/dependencies.gradle'
That's it! Now you can share common declarations for versions name names of dependencies.