warning: [deprecation] (shouldOverrideUrlLoading,onReceivedError,removeAllCookie) in WebViewClient has been deprecated

## System info

Issue occurs on: Android
Plugin version: ^0.3.11
Flutter doctor output:

```
[√] Flutter (Channel stable, v1.12.13+hotfix.9, on Microsoft Windows [Version 10.0.18362.476], locale en-US)
• Flutter version 1.12.13+hotfix.9 at C:\Android\flutter
• Framework revision f139b11009 (5 weeks ago), 2020-03-30 13:57:30 -0700
• Engine revision af51afceb8
• Dart version 2.7.2

[!] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
• Android SDK at C:\Android\sdk\sdk
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-29, build-tools 29.0.2
• ANDROID_HOME = C:\Android\sdk\sdk\platform-tools
• Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
• Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b04)
! Some Android licenses not accepted. To resolve this, run: flutter doctor --android-licenses

[√] Android Studio (version 3.6)
• Android Studio at C:\Program Files\Android\Android Studio
• Flutter plugin version 45.1.1
• Dart plugin version 192.7761
• Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b04)

[!] IntelliJ IDEA Community Edition (version 2019.2)
• IntelliJ at C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2019.2.1
X Flutter plugin not installed; this adds Flutter specific functionality.
X Dart plugin not installed; this adds Dart specific functionality.
• For information about installing plugins, see
https://flutter.dev/intellij-setup/#installing-the-plugins

[√] VS Code (version 1.37.1)
• VS Code at C:\Users\Amr Abd Alkrim\AppData\Local\Programs\Microsoft VS Code
• Flutter extension version 2.25.1

[√] Connected device (1 available)
• G8 • 1090308019017967 • android-arm64 • Android 9 (API 28)

! Doctor found issues in 2 categories.
```

## Logs

```
C:\Android\flutter\.pub-cache\hosted\pub.dartlang.org\flutter_webview_plugin-0.3.11\android\src\main\java\com\flutter_webview_plugin\BrowserClient.java:80: warning: [deprecation] shouldOverrideUrlLoading(WebView,String) in WebViewClient has been deprecated
public boolean shouldOverrideUrlLoading(WebView view, String url) {
^
C:\Android\flutter\.pub-cache\hosted\pub.dartlang.org\flutter_webview_plugin-0.3.11\android\src\main\java\com\flutter_webview_plugin\BrowserClient.java:103: warning: [deprecation] onReceivedError(WebView,int,String,String) in WebViewClient has been deprecated
public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
^
C:\Android\flutter\.pub-cache\hosted\pub.dartlang.org\flutter_webview_plugin-0.3.11\android\src\main\java\com\flutter_webview_plugin\BrowserClient.java:104: warning: [deprecation] onReceivedError(WebView,int,String,String) in WebViewClient has been deprecated
super.onReceivedError(view, errorCode, description, failingUrl);
^
C:\Android\flutter\.pub-cache\hosted\pub.dartlang.org\flutter_webview_plugin-0.3.11\android\src\main\java\com\flutter_webview_plugin\FlutterWebviewPlugin.java:310: warning: [deprecation] removeAllCookie() in CookieManager has been deprecated
CookieManager.getInstance().removeAllCookie();
^
C:\Android\flutter\.pub-cache\hosted\pub.dartlang.org\flutter_webview_plugin-0.3.11\android\src\main\java\com\flutter_webview_plugin\WebviewManager.java:346: warning: [deprecation] removeAllCookie() in CookieManager has been deprecated
CookieManager.getInstance().removeAllCookie();
^
Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
5 warnings
```

```

[√] Flutter (Channel stable, v1.12.13+hotfix.9, on Microsoft Windows [Version 10.0.18362.476], locale en-US)
• Flutter version 1.12.13+hotfix.9 at C:\Android\flutter
• Framework revision f139b11009 (5 weeks ago), 2020-03-30 13:57:30 -0700
• Engine revision af51afceb8
• Dart version 2.7.2

[!] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
• Android SDK at C:\Android\sdk\sdk
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-29, build-tools 29.0.2
• ANDROID_HOME = C:\Android\sdk\sdk\platform-tools
• Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
• Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b04)
! Some Android licenses not accepted. To resolve this, run: flutter doctor --android-licenses

[√] Android Studio (version 3.6)
• Android Studio at C:\Program Files\Android\Android Studio
• Flutter plugin version 45.1.1
• Dart plugin version 192.7761
• Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b04)

[!] IntelliJ IDEA Community Edition (version 2019.2)
• IntelliJ at C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2019.2.1
X Flutter plugin not installed; this adds Flutter specific functionality.
X Dart plugin not installed; this adds Dart specific functionality.
• For information about installing plugins, see
https://flutter.dev/intellij-setup/#installing-the-plugins

[√] VS Code (version 1.37.1)
• VS Code at C:\Users\Amr Abd Alkrim\AppData\Local\Programs\Microsoft VS Code
• Flutter extension version 2.25.1

[√] Connected device (1 available)
• G8 • 1090308019017967 • android-arm64 • Android 9 (API 28)

! Doctor found issues in 2 categories.

```

Bắt đầu với BrowserClient.java, FlutterWebviewPlugin.java và WebviewManager.java, xác định các method được nêu trong các cảnh báo của compiler: shouldOverrideUrlLoading, onReceivedError và removeAllCookie. Build lại Android plugin và xác minh rằng các cảnh báo deprecated này đã biến mất mà không thay đổi behavior của WebView.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.