Andriod WebView 出現 net::ERR_CLEARTEXT_NOT_PERMITTED 錯誤

[問題]
使用 WebView 查看 http 網址，出現 net::ERR_CLEARTEXT_NOT_PERMITTED 錯誤。

[原因]
Android 9 (API level 28) 之後的 API，預設只允許 HTTPS 的加密網站，不允許 HTTP 的明文。

[解決方法]
修改 AndroidManifest.xml，改成可使用明文(cleartext)傳輸，
加上  android:usesCleartextTraffic="true" 設定

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.test">
    <uses-permission android:name="android.permission.INTERNET" />
    <application
.....
        android:usesCleartextTraffic="true"
.....>
.....
    </application>
</manifest>

參考：

• https://developer.android.com/guide/topics/manifest/application-element#usesCleartextTraffic
  <application>  |  Android Developers
  
• https://stackoverflow.com/questions/45940861/android-8-cleartext-http-traffic-not-permitted
  Android 8: Cleartext HTTP traffic not permitted - Stack Overflow
• https://developer.android.com/training/articles/security-config#CleartextTrafficPermitted
  Network security configuration  |  Android Developers
• https://android-developers.googleblog.com/2016/07/changes-to-trusted-certificate.html
  Android Developers Blog: Changes to Trusted Certificate Authorities in Android Nougat