[2/2] OmniGears: Upgrade network traffic meter

*Implements list preferences where applicable to simplify the settings
*Add the ability to track Upstream, Downstream, or both
*Add the option to change the frequency with which it updates

PS3: add a config to disable network traffic meter
PS6: Update strings and check if stats are supported automatically

Base: https://gerrit.omnirom.org/#/c/5129/

PS8
use TrafficStats.UNSUPPORTED constant
use value as summary for list preference

PS10
use this to cleanup some of the other settings nor now

Change-Id: I778d981fb7a57d07127608f99d45936a55ae4898