chore(firebase_crashlytics): Update example app to use Firebase app initialization from native service files.

[russellwheatley]

Description

Crashlytics still requires the Firebase APP_ID in native service file according to issue reports.
There is also another problem running from runZonedGuarded as the initialization is occurring too late and causes the app to throw an exception.

So Crashlytics still needs native initialization for now.

Related Issues

none.

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Breaking Change

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[Salakar]

As discussed internally lets park this in favour of #8157 and invertase/flutterfire_cli#43 - may not be required after these + Android equivalent