Remove MapStyleStateDelegate, use native isStyleLoaded.

[pengdev]

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Summary of changes

This PR remove MapStyleStateDelegate, use native isStyleLoaded.

User impact (optional)

[pengdev]

Closing as the native API behaves differently than the platform cached isStyleLoaded. i.e. the native isStyleLoaded will become false when the new runtime source/layer is added to the style.

We should keep using the platform cached isStyleLoaded for now.