[Code health] Migrate to built-in Kotlin

[shobhitagarwal1612]

After upgrading gradle to 9.0 (#3538), we started seeing a warning to migrate kotlin android plugin to built-in kotlin support.

https://developer.android.com/build/migrate-to-built-in-kotlin

[shobhitagarwal1612]

When I performed the migration, the build failed with the following error:

> Failed to apply plugin 'com.google.protobuf'.
   > Cannot cast object 'com.android.build.gradle.internal.dsl.ApplicationExtensionImpl$AgpDecorated_Decorated@22c97f91' with class 'com.android.build.gradle.internal.dsl.ApplicationExtensionImpl$AgpDecorated_Decorated' to class 'com.android.build.gradle.BaseExtension'

This error happens because the current version of the protobuf-gradle-plugin (version 0.9.6) is not fully compatible with AGP 9.0's new DSL when kotlin-android is removed. It attempts to cast an internal AGP 9.0 object to the deprecated BaseExtension class, resulting in a ClassCastException.

According to the official protobuf-gradle-plugin GitHub issues (specifically #787), this is a known issue that prevents the usage of built-in Kotlin in AGP 9.0 alongside the Protobuf plugin.

[kenstershiro]

Blocked by protobuf plugin issue above, Parking for now