fix(messaging,android): fix call race that could happen when using requestPermission

[Lyokone]

Description

Fixes an Android crash in firebase_messaging when concurrent requestPermission() calls race through the native permission manager. The fix synchronizes the in-flight permission state and stored callback so only one OS permission request can start at a time, and the callback cannot be overwritten while a request is pending.

No automated test was added because FlutterFire CI does not currently run Android plugin JVM tests, and the existing integration test setup cannot reliably drive the Android runtime permission dialog. This was verified with the Android example build plus existing Dart analysis/tests.

Related Issues

• Fixes DuplicateTaskCompletionException crash in requestPermissions() on Android due to unsynchronized requestInProgress flag #18246

Checklist

• 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

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

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[cabljac]

lgtm