Manually wire up copyFlutterAssets task

[markushi]

📜 Description

Flutter isn't using the transform API to add generated sources, and thus breaks our Android transform API approach.
Simply wire up the task manually until flutter will eventually switch to the transform API as well.

More details on the overall problem: https://www.notion.so/sentry/ProGuard-Mapping-File-upload-sentry-vs-the-rest-e90c1fd768a94bfdb4e0a9ad0ac80503?pvs=4#42b4620bf43349ae9fd091b7c36dda0a

💡 Motivation and Context

Fixes #701

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

[stefanosiano]

nice!

[romtsn]

flutter is shiet