buildTypes {
    applicationVariants.all { variant ->
        def oldFile = variant.outputs.outputFile.get(0)
        def newFile = new File(
                oldFile.parent,
                oldFile.name.replace(".apk", "-" + defaultConfig.versionName + ".apk"))

        variant.outputs.outputFile.set(0, newFile)
}

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

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

applicationVariants.all { variant ->
    variant.outputs.all { output ->
        outputFileName = "myapp-${variant.versionName}-${variant.name}.apk"
    }
}