Let listening probes collect continuously on AC power

[MatthewGerber]

@clmckinley This is an idea that came up in a meeting today. Previously, we added polling based on AC power connect events. Now we'd like to enhance listening probes in a similar fashion. When the user plugs the device into AC power, allow all listening probes to collect data uninterrupted, even when the app is backgrounded. On Android, this simply amounts to calling SensusServiceHelper.KeepDeviceAwake (when AC power is connected) and then SensusServiceHelper.LetDeviceSleep (when AC power is disconnected). We don't have a great way to do this on iOS. The only thing I can imagine doing is enabling the GPS probe, which should ensure uninterrupted data streams for some of the probes. I would start with Android and figure out iOS later.