docs: update kotlin integration with Existing Apps and Fragment

[hurali97]

Description:

While following the docs for integrating React Native within a Kotlin App, the docs appeared to be outdated as the build configurations described in the docs were failing. The same case was with the Kotlin integration code snippets.

This might have happened due to the docs for brownfield integration not being up-to-date with latest RN versions.

Hence, I have created this PR after figuring out the build configurations that worked and the updated code snippets. I took some help from build gradle files from react-native repo in order to use the correct build configurations.

[netlify[bot]]

✅ Deploy Preview for react-native ready!

Name	Link
🔨 Latest commit	c7ed54fcef2566b58fbca70f964e3e15bebaffa7
🔍 Latest deploy log	https://app.netlify.com/sites/react-native/deploys/66d336f6b030070008ec24f0
😎 Deploy Preview	https://deploy-preview-4204--react-native.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.