NetworkTraffic: Prevent network callback exception on theme change

* On theme change, status bar icons are re-added.
  This re-called registerNetworkCallback everytime leading to
  "too many requests" and systemui crash.
* Prevent it by register callback when attached to window.
  Also, unregister callback when detaching.

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>