feat(messaging): change usage of Platform.isIos to defaultTargetPlatform to allow mocking platform more easily

[fachrifaul]

Description

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Hard to mock Platform, need to change to defaultTargetPlatform

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[russellwheatley]

@fachrifaul - You need to remove this import from the same file you changed: https://github.com/firebase/flutterfire/actions/runs/7626531820/job/20773125499?pr=12183#step:5:241

[fachrifaul]

@fachrifaul - You need to remove this import from the same file you changed: https://github.com/firebase/flutterfire/actions/runs/7626531820/job/20773125499?pr=12183#step:5:241

Done

[ellet0]

defaultTargetPlatform doesn't care if the platform is running on web browser or not, it could be TargetPlatform.iOS but on web browser, we need to handle this case as well