[firebase_remote_config] Fix Future already completed RemoteConfig.instance

[IchordeDionysos]

Description

Fix Future already completed for calling multiple RemoteConfig.instance initially in parallel

Related Issues

flutter/flutter#38060

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Breaking Change

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[collinjackson]

Migrated to firebase/flutterfire#28

Note: It seems the plugin was working fine and this behavior is covered by a test, we just need to remove the spurious error message.