[ios] fix asset url not found when loading app extension (flutter/engine#46073)
In b3fc48b91b, I refactored the assetsPath to use NSURL. It turns out that when the app bundle is not loaded (during launching app exgtension), the assetURL will return nil using the `URLForResource`, but the `pathForResource` successfully returns the raw path.
This PR reverts back to the raw path solution.
part of https://github.com/flutter/flutter/issues/124287
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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