fix(messaging,android): skip onMessageOpenedApp for terminated notification taps

[SelaseKay]

Description

On Android, tapping an FCM notification from a terminated state stored the message for getInitialMessage() and invoked onMessageOpenedApp. Apps that subscribe to the stream early (for example in main() before runApp()) therefore handled the same tap twice, which does not match the Dart docs or iOS.

onAttachedToActivity (new Activity / terminated launch) now stores the message only. onNewIntent (existing Activity / background resume) still fires onMessageOpenedApp.

Related Issues

• Fixes [firebase_messaging][Android] Terminated launch delivers the same message to both getInitialMessage() and onMessageOpenedApp (still reproduces on main; iOS already guards against this) #18661

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

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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