Update android embedder design doc to rfc

[mboetger]

Update the link for the Android Embedder Design doc to point at the new RFC.

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[gemini-code-assist]

Code Review

This pull request updates the redirect destination for /go/flutter-android-embedder-api-migration in sites/docs/firebase.json from a Google Document to a GitHub RFC pull request. There are no review comments, and I have no feedback to provide.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 8635d5d):

https://flutter-docs-prod--docs-pr13927-move-from-doc-to-rfc-qo0rzpur.web.app