gradle_andr_version_code.groovy

// add method to get the versionCode from cli param
// eg. `gradle tasks -PversionCode=999`

/**
 * Get the version code from command line param
 * 
 * @return int If the param -PversionCode is present then return int value or -1
 */
def getVersionCode = { ->

    def code = project.hasProperty('versionCode') ? versionCode.toInteger() : -1
    println "VersionCode is set to $code"
    return code
}

// add to defaultConfig

android {
    defaultConfig {

        versionCode getVersionCode()

        //...

    }
}