Implement Android WebView api with pigeon (Java portion)

packages/webview_flutter/webview_flutter_android/android/build.gradle

@@ -42,6 +42,10 @@ android {
         testImplementation 'androidx.test:core:1.3.0'
     }
 
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
 
     testOptions {
         unitTests.includeAndroidResources = true

packages/webview_flutter/webview_flutter_android/android/src/main/java/io/flutter/plugins/webviewflutter/DownloadListenerHostApiImpl.java

@@ -0,0 +1,44 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package io.flutter.plugins.webviewflutter;
+
+import android.webkit.DownloadListener;
+import io.flutter.plugins.webviewflutter.GeneratedAndroidWebView.DownloadListenerFlutterApi;
+
+class DownloadListenerHostApiImpl implements GeneratedAndroidWebView.DownloadListenerHostApi {
+  private final InstanceManager instanceManager;
+  private final DownloadListenerCreator downloadListenerCreator;
+  private final GeneratedAndroidWebView.DownloadListenerFlutterApi downloadListenerFlutterApi;
+
+  static class DownloadListenerCreator {
+    DownloadListener createDownloadListener(
+        Long instanceId, DownloadListenerFlutterApi downloadListenerFlutterApi) {
+      return (url, userAgent, contentDisposition, mimetype, contentLength) ->
+          downloadListenerFlutterApi.onDownloadStart(
+              instanceId, url, userAgent, contentDisposition, mimetype, contentLength, reply -> {});
+    }
+  }
+
+  DownloadListenerHostApiImpl(
+      InstanceManager instanceManager,
+      DownloadListenerCreator downloadListenerCreator,
+      DownloadListenerFlutterApi downloadListenerFlutterApi) {
+    this.instanceManager = instanceManager;
+    this.downloadListenerCreator = downloadListenerCreator;
+    this.downloadListenerFlutterApi = downloadListenerFlutterApi;
+  }
+
+  @Override
+  public void create(Long instanceId) {
+    final DownloadListener downloadListener =
+        downloadListenerCreator.createDownloadListener(instanceId, downloadListenerFlutterApi);
+    instanceManager.addInstance(downloadListener, instanceId);
+  }
+
+  @Override
+  public void dispose(Long instanceId) {
+    instanceManager.removeInstance(instanceId);
+  }
+}

packages/webview_flutter/webview_flutter_android/android/src/main/java/io/flutter/plugins/webviewflutter/FlutterWebViewClient.java

@@ -104,7 +104,7 @@ boolean shouldOverrideUrlLoading(WebView view, String url) {
       return false;
     }
     // This version of shouldOverrideUrlLoading is only invoked by the webview on devices with
-    // webview versions  earlier than 67(it is also invoked when hasNavigationDelegate is false).
+    // webview versions earlier than 67(it is also invoked when hasNavigationDelegate is false).
     // On these devices we cannot tell whether the navigation is targeted to the main frame or not.
     // We proceed assuming that the navigation is targeted to the main frame. If the page had any
     // frames they will be loaded in the main frame instead.