[Firestore] Update setup instructions

[dlarocque]

Refactors Firestore README to fix setup inaccuracies I found when setting up my local dev environment and improves the documentation for running integration tests.

• Corrected google-services.json path: The previous instruction for placing google-services.json at the root dir was incorrect, and caused the tests to fallback to using a dummy config, which caused my tests to fail. After moving the file to firebase-firestore/google-services.json the tests passed for me.
• Documented hidden CLI flags (-PtargetBackend, -PbackendEdition, -PtargetDatabaseId)
• Removed IDE-specific Run Configurations, which I couldn't find in Android Studio anywhere. I don't think they exist anymore.
• Moved prerequisites section to the very of the testing section, instead of being buried below build commands

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[dlarocque]

/gemini review

[gemini-code-assist]

Code Review

This pull request significantly improves the clarity and accuracy of the Firestore setup instructions. The refactoring of the README makes it much more organized and easier for developers to follow. Documenting the previously hidden CLI flags is also a very helpful addition. I've identified a couple of minor opportunities for improvement and one important inconsistency regarding the google-services.json file path that should be addressed to avoid confusion.