refactor(firestore,android): migrate native implementation to Kotlin

[SelaseKay]

Description

Migrates the cloud_firestore Android implementation from Java to Kotlin while preserving the existing Pigeon HostApi and EventChannel behavior.

• HostApi implementation is now Kotlin (FlutterFirebaseFirestorePlugin)
• Switches Pigeon codegen from javaOut to kotlinOut
• Reparents the generated Pigeon codec onto FlutterFirebaseFirestoreMessageCodec so EventChannels can still emit InternalDocumentSnapshot / query snapshots
• Updates the Android Gradle configuration to compile Kotlin sources across supported AGP versions
• Unregisters EventChannels on stream onCancel (query, document, loadBundle, snapshotsInSync), matching the Apple review fix for the same listener leak
• Pipeline expression parsing still uses the typed Android SDK APIs (left.equal(right), etc.)

This PR is not stacked on #18607 (Swift). Both PRs touch pigeons/messages.dart; rebase whichever lands second.

:cloud_firestore:assembleDebug compiled successfully. The example app was not fully assembled locally (google-services.json is not in the tree). Firestore integration tests were not run.

Related Issues

None

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

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