Fix bugs in TrafficRecorder

Addresses #220.

mobile/src/main/java/be/mygod/vpnhotspot/net/monitor/TrafficRecorder.kt

@@ -130,6 +130,7 @@ object TrafficRecorder {
     }
     fun update(timeout: Boolean = false) {
         synchronized(this) {
+            unscheduleUpdateLocked()
             if (records.isEmpty()) return
             val timestamp = System.currentTimeMillis()
             if (!timeout && timestamp - lastUpdate <= 100) return
@@ -141,7 +142,6 @@ object TrafficRecorder {
                 SmartSnackbar.make(e).show()
             }
             lastUpdate = timestamp
-            updateJob = null
             scheduleUpdateLocked()
         }
     }