Update AR Foundation instructions and fix missing XRmanifest.androidlib

[timbotimbo]

Description

If your Unity project uses XR Plugin management 4.4 or higher, you get an error when building on android. https://github.com/juicycleff/flutter-unity-view-widget/issues/844

Project with path 'xrmanifest.androidlib' could not be found in project ':unityLibrary

You need to add an include to settings.gradle to handle this.

Changes

• Document including xrmanifest.androidlib in the readme.

include ":unityLibrary:xrmanifest.androidlib"

• Update outdated AR Foundation setup in the readme. You don't need to manually include all the .aar files in gradle anymore.
• Android export now tries to automatically include xrmanifest.androidlib in settings.gradle

Type of Change

• [ ] ✨ New feature (non-breaking change which adds functionality)
• [x] 🛠️ Bug fix (non-breaking change which fixes an issue)
• [ ] ❌ Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🧹 Code refactor
• [ ] ✅ Build configuration change
• [x] 📝 Documentation
• [ ] 🗑️ Chore

This PR is somewhat related to https://github.com/juicycleff/flutter-unity-view-widget/pull/908, as ARFoundation will crash on recent unity versions without that fix.

[CristovoXDGM]

@juicycleff please merge this, we need updates for this current version of the plugin

[timbotimbo]

Replaced with #960 to also update vuforia.