custom versionName with versionCode + timestamp, in release apk filename

build.gradle

android {
    ....
    defaultConfig {
        ...
        versionCode 3
        versionName versionCode + "-" + getTimestamp()
    }
    ...
    ...
    applicationVariants.all { variant ->
        println "********* " + variant.description + " **********";
        def apkName = "MyAppName";
        apkName += "-" + android.defaultConfig.versionName;
        if (variant.buildType.name == "release") {
            variant.outputs.each { output ->
                output.outputFile = new File(
                        output.outputFile.parent,
                        apkName + ".apk");
                println "********* " + output.outputFile + " **********";
            }
        }
    }
}

...
...

def getTimestamp() {
    def date = new Date()
    return date.format('yyyyMMdd.HHmm')
}