Migrate build logic from Groovy to Kotlin

[uhooi]

Overview

Replace build.gradle with build.gradle.kts.

Reference

• https://docs.gradle.org/current/userguide/migrating_from_groovy_to_kotlin_dsl.html
• https://proandroiddev.com/the-new-way-of-writing-build-gradle-with-kotlin-dsl-script-8523710c9670