Improve network bandwidth calculations

[PaulMaddox]

Currently the network bandwidth calculations in NetworkUsageController.java could do with some averaging to prevent spiky behaviour, especially on higher latency connections.

Currently it works by getting the traffic counters from /proc/net/dev every second (roughly, depends on users connection). It then calculates the bandwidth by checking how many milliseconds since the last check, and dividing the different in bytes between the counters by the time difference.

I think this would be better if it took an average over either:

• The last x number of checks
• The last minute