Use jinja2 to template webview manifest files.

android_webview/apk/java/AndroidManifest.xml

 <?xml version="1.0" encoding="utf-8"?>
 <!--
   Copyright (c) 2012 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.
 -->
 
 <!-- Please keep this in sync with the downstream android_webview manifest.
 -->
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
-    package="com.android.webview">
-    <uses-sdk android:minSdkVersion="21"
-              android:targetSdkVersion="21">
+    package="{{package|default('com.android.webview')}}">
+    <uses-sdk android:minSdkVersion="{{minsdk|default(21)}}"
+              android:targetSdkVersion="{{targetsdk|default(21)}}">
     </uses-sdk>
 
     <uses-feature android:name="android.hardware.touchscreen"
             android:required="false"/>
 
     <application android:label="Android System WebView"
         android:icon="@drawable/icon_webview"
         android:multiArch="true">
         <activity android:name="com.android.webview.chromium.LicenseActivity"
                   android:label="@string/license_activity_title"
                   >
             <intent-filter>
                 <action android:name="android.settings.WEBVIEW_LICENSE" />
                 <category android:name="android.intent.category.DEFAULT" />
             </intent-filter>
             <meta-data android:name="com.android.settings.PRIMARY_PROFILE_CONTROLLED"
                        android:value="true" />
         </activity>
         <provider android:name="com.android.webview.chromium.LicenseContentProvider"
                   android:exported="true"
                   android:authorities="com.android.webview.chromium.LicenseContentProvider" />
     </application>
 </manifest>