add app description to manifest to support p2p distribution

This allows other methods of distributing APKs to also include the
description text, like the FDroid local repo swapping.

AndroidManifest.xml

@@ -18,6 +18,7 @@
 
     <application android:name=".OrbotApp" android:icon="@drawable/ic_launcher"
      android:label="@string/app_name" 
+     android:description="@string/app_description"
      android:configChanges="locale|orientation|screenSize"
       android:theme="@style/Theme.AppCompat" 
       android:allowBackup="false"

res/values/strings.xml

@@ -1,6 +1,7 @@
 <?xml version='1.0' encoding='UTF-8'?>
 <resources>
   <string name="app_name">Orbot</string>
+  <string name="app_description">Orbot is a free proxy app that empowers other apps to use the internet more securely. Orbot uses Tor to encrypt your Internet traffic and then hides it by bouncing through a series of computers around the world. Tor is free software and an open network that helps you defend against a form of network surveillance that threatens personal freedom and privacy, confidential business activities and relationships, and state security known as traffic analysis.</string>
   <string name="internal_web_url">http://orbot/</string>
   <string name="default_web_url">http://check.torproject.org</string>
   <string name="secure_default_web_url">https://check.torproject.org</string>