Disable Tor's connection padding
This branch disables Tor's connection padding when we're using mobile data or running on battery.
Connection padding is a traffic analysis countermeasure that's enabled by default in Tor 0.3.4 if the guard supports it. Measurements show that connection padding roughly doubles our bandwidth usage when running a hidden service with no contacts.
The branch adds a new BatteryManager that reports whether we're running on battery.