Gradle: Migrate top-level build files to Kotlin DSL

Migrate the top-level build files to the new Kotlin DSL and use the new
pluginManagement block instead of the old buildscript.

app/build.gradle

@@ -1,10 +1,10 @@
 import java.nio.file.Files
 
 plugins {
-    id 'com.android.application'
-    id 'com.google.protobuf'
-    id 'kotlin-android'
-    id 'kotlin-parcelize'
+    alias(libs.plugins.android.application)
+    alias(libs.plugins.kotlin.android)
+    alias(libs.plugins.kotlin.parcelize)
+    alias(libs.plugins.protobuf)
 }
 
 tasks.withType(Test).configureEach {