custom.build.gradle

ext {
    appVersionName = '1.0'
    appPackageId = '<YourApplicationID>'
}

android {
    compileSdkVersion 25
    buildToolsVersion "25.0.2"

    defaultConfig {
        applicationId "$appPackageId"
        minSdkVersion 19
        targetSdkVersion 25
        versionCode 1
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    signingConfigs {
        debug {
            storeFile file("<YourDebugKeyStoreLocation>")
        }
        release {
            storeFile file("<YourReleaseKeyStoreLocation>")
            storePassword "<YourReleaseKeyStorPassword>"
            keyAlias "<YourKeyAlias>"
            keyPassword "<YourKeyAliasPassword>"
        }
    }

    buildTypes {
        debug {
            minifyEnabled true
            useProguard false
            proguardFiles getDefaultProguardFile ('proguard-android.txt'), 'proguard-rules.pro'
            signingConfig signingConfigs.debug
        }
        release {
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
            signingConfig signingConfigs.release
        }
    }

    productFlavors {
        internal {
            applicationId "$appPackageId" + ".internal"
            versionName "i$appVersionName"
        }

        production {
            applicationId "$appPackageId"
            versionName "$appVersionName"
        }
    }

    applicationVariants.all { variant ->
        variant.outputs.each { output ->
            //noinspection GroovyAssignabilityCheck
            output.outputFile = new File(output.outputFile.parent,
                    output.outputFile.name.replace(".apk", "-${variant.versionName}.apk"))
        }
    }
}