[global_unity_controller] Fix dispose not called

[Ortes]

Description

Call dispose for both FlutterUnityController and FlutterUnityPlatform

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ 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
• 📝 Documentation
• 🗑️ Chore

[timbotimbo]

Given the changes and the fact that this merges into feat/global_unity_controller, this seems specific to that branch.
I'll add a mention to the original pull request and update the title, to make this easier to see.

Looks like this fixes regression number 1 mentioned here