For loading the game on the android app, we download and cache the whole game bundle locally to reduce the download time for subsequent game loads. We leverage the request interceptor functionality of WebView to intercept the network request and then use the WebViewAssetLoader to load the response for the intercepted request from internal storage.
Can I ask why you are doing this in the first place? Construct's built-in Android export option already gives you an Android app with all the resources bundled, so it works offline, and you don't need to download anything from the Internet, do anything else to manage offline support, or write your own network request interceptor. Construct's Android export has quite a lot of complicated compatibility code to work around all these types of issues, and so if you don't use Construct's Android export, you're going to have a long path of reinventing all these difficult compatibility headaches, such as this one, and probably several more you will encounter in future.
Anyway, this specific problem should be simple to solve: as the error message says you just need to make sure the returned MIME type is application/javascript for .js files. A quick Google shows that you can set this on the returned WebResourceResponse via setMimeType. However as I said, this will probably look like it works until you run in to the next compatibility problem which you have to add further hacks for...