Unregister preference change callback in onDestroy instead of onPause

Fix #496

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

@@ -104,7 +104,6 @@ class Shadowsocks
   var progressDialog: ProgressDialog = _
   var progressTag = -1
   var state = State.STOPPED
-  var prepared = false
   var currentProfile = new Profile
   var vpnEnabled = -1
   var trafficCache: String = _
@@ -402,7 +401,6 @@ class Shadowsocks
   protected override def onPause() {
     super.onPause()
     ShadowsocksApplication.profileManager.save
-    prepared = false
   }
 
   private def stateUpdate() {
@@ -517,7 +515,6 @@ class Shadowsocks
 
   override def onActivityResult(requestCode: Int, resultCode: Int, data: Intent) = resultCode match {
     case Activity.RESULT_OK =>
-      prepared = true
       serviceStart()
     case _ =>
       cancelStart()

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

@@ -166,18 +166,16 @@ class ShadowsocksSettings extends PreferenceFragment with OnSharedPreferenceChan
     isProxyApps.setChecked(ShadowsocksApplication.settings.getBoolean(Key.isProxyApps, false))  // update
   }
 
-  override def onPause {
-    super.onPause()
+  override def onDestroy {
+    super.onDestroy()
     ShadowsocksApplication.settings.unregisterOnSharedPreferenceChangeListener(this)
   }
 
   def onSharedPreferenceChanged(sharedPreferences: SharedPreferences, key: String) = key match {
-    case Key.isNAT =>
-      activity.handler.post(() => {
-        activity.deattachService
-        activity.attachService
-      })
-      setEnabled(enabled)
+    case Key.isNAT => activity.handler.post(() => {
+      activity.deattachService
+      activity.attachService
+    })
     case _ =>
   }