fix(messaging, android): stop doing disk I/O on the main thread in the message broadcast receiver

[Lyokone]

Description

FlutterFirebaseMessagingReceiver.onReceive() runs on the main thread and wrote every notification message to SharedPreferences; when a broadcast receiver finishes, Android blocks the main thread until pending SharedPreferences writes are flushed to disk (QueuedWork.waitToFinish), causing ANRs under I/O pressure. The receiver now hands the store write and message dispatch to a background executor via goAsync(), so no disk I/O or binder calls happen on the main thread; the in-memory notification map update stays on the main thread, where it is read.

Verified on an Android emulator with synthetic FCM broadcasts (adb root + am broadcast to the receiver): foreground delivery to onMessage, background delivery via FlutterFirebaseMessagingBackgroundService, and message persistence to the store all behave identically before and after the change, and the firebase_messaging e2e suite passes. The ANR itself is a timing condition that cannot be asserted by the existing test infrastructure (no native Android unit tests in the repo, and an FCM push cannot be triggered from the Dart integration tests).

Related Issues

• Fixes [firebase_messaing]: android.os.MessageQueue.nativePollOnce (Native method) #12832

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). (Not testable with current infra: ANRs are timing conditions and the repo has no native Android unit tests; behavior parity was verified manually on an emulator — see description.)
• 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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