Update MojoShell's target APK version to 23.

shell/android/apk/AndroidManifest.xml

 <?xml version="1.0" encoding="utf-8"?>
 
 <!-- Copyright 2013 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.
  -->
 
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="org.chromium.mojo.shell">
 
-    <uses-sdk android:minSdkVersion="14" android:targetSdkVersion="21" />
+    <uses-sdk android:minSdkVersion="14" android:targetSdkVersion="23" />
     <uses-permission android:name="android.permission.INTERNET"/>
     <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/>
 
     <application android:name="MojoShellApplication"
             android:label="Mojo Shell">
         <meta-data android:name="com.google.android.gms.version"
                    android:value="@integer/google_play_services_version" />
         <activity android:name="MojoShellActivity"
                   android:launchMode="singleTask"
                   android:theme="@android:style/Theme.Holo.Light.NoActionBar"
                   android:configChanges="orientation|keyboardHidden|keyboard|screenSize"
                   android:hardwareAccelerated="true">
             <intent-filter>
                 <action android:name="android.intent.action.VIEW"/>
             </intent-filter>
         </activity>
     </application>
 
 </manifest>