This class is no longer needed now that Google provides WebViewAssetLoader (see https://developer.android.com/reference/kotlin/androidx/webkit/WebViewAssetLoader)

WebViewServer.kt

/*
MIT License

Copyright (c) 2019 Erik Hellman

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

@file:Suppress("unused")

package se.hellsoft.webviewserver

import android.content.res.AssetManager
import android.net.Uri
import android.webkit.WebResourceRequest
import android.webkit.WebResourceResponse
import java.io.ByteArrayInputStream
import java.io.File
import java.io.FileInputStream
import java.io.InputStream

/**
 * This class will act as a web server for intercepted request from a `WebView`. Simply implement your own
 * `WebViewClientCompat` and assign it as the `webViewClient` of your `WebView` and call `handleWebViewRequest()` from
 *  `WebViewClient.shouldInterceptRequest()`. 
 *  
 * Just add your own `RequestHandler` for every response you want to return. Two ready-made implementations
 * of `RequestHandler` are provided: `FileRequestHandler` and `AssetRequestHandler`. See respective class for details.
 *
 * If no matching `RequestHandler` is found, a default 404 response will be returned.
 */
class WebViewServer {
    private val requestHandlers = mutableListOf<RequestHandler>()

    fun addRequestHandler(requestHandler: RequestHandler) {
        requestHandlers += requestHandler
    }

    fun handleWebViewRequest(webResourceRequest: WebResourceRequest): WebResourceResponse {
        val request = Request.fromWebResourceRequest(webResourceRequest)

        for (requestHandler in requestHandlers) {
            if (requestHandler.shouldHandleRequest(request)) {
                return requestHandler.handleRequest(request).toWebResourceResponse()
            }
        }

        return NOT_FOUND_RESPONSE.toWebResourceResponse()
    }

    companion object {
        val NOT_FOUND_RESPONSE = Response(
            404,
            "Not found",
            emptyMap(),
            "text/plain",
            "UTF-8",
            ByteArrayInputStream(byteArrayOf())
        )
    }
}

data class Request(val url: String, val method: String, val headers: Map<String, String>) {
    companion object {
        fun fromWebResourceRequest(webResourceRequest: WebResourceRequest): Request {
            return Request(
                webResourceRequest.url.toString(),
                webResourceRequest.method,
                webResourceRequest.requestHeaders
            )
        }
    }
}

data class Response(
    val statusCode: Int,
    val reasonPhrase: String,
    val responseHeaders: Map<String, String>,
    val mimeType: String,
    val encoding: String,
    val data: InputStream
) {
    fun toWebResourceResponse(): WebResourceResponse {
        return WebResourceResponse(mimeType, encoding, statusCode, reasonPhrase, responseHeaders, data)
    }
}

interface RequestHandler {
    fun shouldHandleRequest(request: Request): Boolean

    fun handleRequest(request: Request): Response
}

/**
 * Returns a response containing the content of the specified `File`. Note that the content of the file 
 * is cached in memory, so beware of loading too large files using this class. For very large files, 
 * please provide your own `RequestHandler` that streams the content to the `WebView` instead.
 */
open class FileRequestHandler(
    file: File,
    private val requestPath: String,
    private val mimeType: String,
    private val encoding: String
) : RequestHandler {
    private val bytes: ByteArray

    init {
        bytes = FileInputStream(file).use {
            return@use it.readBytes()
        }
    }

    override fun shouldHandleRequest(request: Request): Boolean {
        val uri = Uri.parse(requestPath)
        return uri.path == requestPath
    }

    override fun handleRequest(request: Request): Response {
        return Response(200, "OK", emptyMap(), mimeType, encoding, ByteArrayInputStream(bytes))
    }
}

/**
 * Returns a response containing the content of the specified Android asset. Note that the content of the file
 * is cached in memory, so beware of loading too large files using this class. For very large files,
 * please provide your own `RequestHandler` that streams the content to the `WebView` instead.
 */
open class AssetRequestHandler(
    assetManager: AssetManager,
    assetPath: String,
    private val requestPath: String,
    private val mimeType: String,
    private val encoding: String
) : RequestHandler {
    private val bytes: ByteArray

    init {
        val inputStream = assetManager.open(assetPath)
        bytes = inputStream.use {
            return@use it.readBytes()
        }
    }

    override fun shouldHandleRequest(request: Request): Boolean {
        val uri = Uri.parse(requestPath)
        return uri.path == requestPath
    }

    override fun handleRequest(request: Request): Response {
        return Response(200, "OK", emptyMap(), mimeType, encoding, ByteArrayInputStream(bytes))
    }
}

Great!
I have translated it into java, in case anyone needs it: https://gist.github.com/rahulk11/dd9fa58bac561c69bff704a24db28747

Hey all! Note that this is no longer needed now that Google provides the WebViewAssetLoader (see https://developer.android.com/reference/kotlin/androidx/webkit/WebViewAssetLoader).

Thanks a lot.