Partial rewrite of traffic indicator

- lower system impact: nothing runs when screen is off (previous version ran continuously unless the network speed indicator was disabled
- show summary of traffic when traffic stops (number of bytes in the current "burst")
- better formatting of the indicator

Remaining to improve:

- stop the indicator when the status bar is hidden (can't figure out how to get notifications when it does get hidden)

Change-Id: Iae248454a9f66979eb6c8148ac91faf820764831