add backup agent support

src/main/AndroidManifest.xml

@@ -19,8 +19,12 @@
             android:name=".ShadowsocksApplication"
             android:hardwareAccelerated="true"
             android:icon="@drawable/ic_launcher"
+            android:backupAgent=".ShadowsocksBackupAgent"
             android:label="@string/app_name">
 
+        <meta-data android:name="com.google.android.backup.api_key"
+            android:value="AEdPqrEAAAAI_zVxZthz2HDuz9toTvkYvL0L5GA-OjeUIfBeXg" />
+
         <activity
                 android:name=".Shadowsocks"
                 android:label="@string/app_name"

src/main/scala/com/github/shadowsocks/Shadowsocks.scala

@@ -65,6 +65,7 @@ import org.jraf.android.backport.switchwidget.Switch
 import android.content.pm.{PackageInfo, PackageManager}
 import android.net.{Uri, VpnService}
 import android.webkit.{WebViewClient, WebView}
+import android.app.backup.BackupManager
 
 object Shadowsocks {
 
@@ -513,6 +514,7 @@ class Shadowsocks
     super.onDestroy()
     Crouton.cancelAllCroutons()
     unregisterReceiver(receiver)
+    (new BackupManager(this)).dataChanged()
   }
 
   def reset() {

src/main/scala/com/github/shadowsocks/ShadowsocksBackupAgent.scala

@@ -0,0 +1,19 @@
+package com.github.shadowsocks
+
+import android.app.backup.{SharedPreferencesBackupHelper, BackupAgentHelper}
+
+class ShadowsocksBackupAgent extends BackupAgentHelper {
+
+  // The names of the SharedPreferences groups that the application maintains.  These
+  // are the same strings that are passed to getSharedPreferences(String, int).
+  val PREFS_DISPLAY = "com.github.shadowsocks_preferences"
+
+  // An arbitrary string used within the BackupAgentHelper implementation to
+  // identify the SharedPreferencesBackupHelper's data.
+  val MY_PREFS_BACKUP_KEY = "com.github.shadowsocks"
+
+  override def onCreate() {
+    val helper = new SharedPreferencesBackupHelper(this, PREFS_DISPLAY)
+    addHelper(MY_PREFS_BACKUP_KEY, helper)
+  }
+}