setup Tor at app start, and config immediately when pref is changed

This adds an Application subclass to get the onCreate() method, which is
called once at the first start up of the app, before any Activity starts.
Tor is configured there to ensure it is setup before anything happens.

This also moves the "Use Tor" pref listener to a more appropriate place.

app/src/main/AndroidManifest.xml

@@ -7,6 +7,7 @@
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
     <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
     <application
+        android:name=".App"
         android:allowBackup="true"
         android:icon="@mipmap/ic_launcher"
         android:logo="@mipmap/ic_launcher"