fix(core, android): synchronize access to registeredPlugins in FlutterFirebasePluginRegistry

[SelaseKay]

Description

Fixes an intermittent Android ConcurrentModificationException during Firebase.initializeApp() by synchronizing access to the FlutterFire plugin registry.

The registry uses a shared WeakHashMap of registered plugins. During startup, firebase_core iterates that map to collect plugin constants, while plugins may still be registering. This change synchronizes registry writes and snapshots the registry before iteration, avoiding races while keeping plugin callback execution outside the lock.

Related Issues

Fixes #18405

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Checklist

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• 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).
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• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.

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