[AW SystemWebViewShell] Increase target_sdk version to 28 (P) and enable cleartext traffic

Update the SystemWebViewShell to target Android P (API level 28).
This is a part of the general effort to update apps to API level 28,
see crbug.com/898190 and b/115499280.

This patch also ensures that cleartextTrafficPermitted is set to
true to ensure backwards compatibility for the SystemWebViewShell
(and to allow cleartext traffic, e.g. using unencrypted http
protocol).

This patch also updates trust-anchors to include system and user
certificates.

BUG=898190

Change-Id: I7adb7dbfce1baf3e7b2cf4860709d10fe295217e
Reviewed-on: https://chromium-review.googlesource.com/c/1296601
Commit-Queue: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: Selim Gurun <sgurun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605057}

android_webview/tools/system_webview_shell/apk/AndroidManifest.xml

@@ -10,7 +10,7 @@
     android:versionCode="1"
     android:versionName="1.0" >
 
-    <uses-sdk android:minSdkVersion="19" android:targetSdkVersion="24" />
+    <uses-sdk android:minSdkVersion="19" android:targetSdkVersion="28" />
 
     <!-- "Normal" permissions which do not require user prompt -->
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
@@ -32,6 +32,7 @@
         android:icon="@drawable/ic_launcher"
         android:label="@string/app_name"
         android:theme="@android:style/Theme.Light"
+        android:networkSecurityConfig="@xml/network_security_config"
         android:debuggable="true" >
         <meta-data android:name="android.webkit.WebView.EnableSafeBrowsing"
             android:value="true" />

android_webview/tools/system_webview_shell/apk/res/xml/network_security_config.xml

+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright 2018 The Chromium Authors. All rights reserved.
+     Use of this source code is governed by a BSD-style license that can be
+     found in the LICENSE file. -->
+
+<network-security-config>
+    <!-- Starting with Android P (API level 28), the default value of
+    isCleartextTrafficPermitted() is false. For the SystemWebViewShell
+    test browser we explicitly set cleartextTrafficPermitted here to
+    preserve functionality. (crbug.com/898190) -->
+    <base-config cleartextTrafficPermitted="true">
+        <trust-anchors>
+            <certificates src="user"/>
+            <certificates src="system"/>
+        </trust-anchors>
+    </base-config>
+</network-security-config>
\ No newline at end of file