Time active

[winsmith]

Register for events. When the app is coming to the foreground, log the time. When the app is going to the background, calculate the time elapsed and send a signal with type "session.end" with "floatValue" the amount of seconds elapsed.

[Jeehut]

This is very much related to our Pirate Metrics topic and could be handled together with that.

[Jeehut]

This is pretty much exactly what we implemented in #230. Closing. 🎉

Note that the signal is named TelemetryDeck.Session.started and the parameter is named TelemetryDeck.Retention.previousSessionSeconds.