SystemUI: NetworkTraffic: Refactor message handling

* In certain network situations (e.g. while changing from wifi to
  cellular) it can happen, that the HashMap is modified while the stats
  are currently being recalculated
* We can prevent this by moving adding and removing of any network
  to the same handler so the callbacks can happen without breaking the
  flow
* Move the existing code in handleMessage into some little helper
  methods so it stays simple and readable

Fixes: https://gitlab.com/LineageOS/issues/android/-/issues/2870
Change-Id: I38c0c27a4f76fab5aeee013beab346c088bbbd59