Fix an issue of NAT mode

If call notify() after startForeground() on the same notification id, the
service becomes not that "foreground" and would be killed by system if the
activity is removed from the recent list.

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

@@ -30,7 +30,6 @@ class ShadowsocksNotification(private val service: BaseService, profileName: Str
   }
   private var lockReceiver: BroadcastReceiver = _
   private var callbackRegistered: Boolean = _
-  private var started: Boolean = _
 
   private val builder = new NotificationCompat.Builder(service)
     .setWhen(0)
@@ -77,9 +76,8 @@ class ShadowsocksNotification(private val service: BaseService, profileName: Str
     show()
   } else if (forceShow) show()
 
-  def show() = if (started) nm.notify(1, builder.build) else {
+  def show() = {
     service.startForeground(1, builder.build)
-    started = true
   }
 
   def destroy() {