Add automatic tracking of app_open lifecycle event

[yalisassoon]

Should populate the app_open schema.

It has been suggested that this should include a fromBackground field to distinguish if the application is opened from the background vs launched from the homescreen. Is that necessary @bogaert and @mhadam? Wouldn't you expect to fire an app_foreground event in the former case? (Once clarified we can update the corresopnding schema in Iglu-Central.

[bogaert]

We don't define a separate open event in our implementations: foreground and background are indeed enough. If it's relevant to know whether it was a cold start or not, perhaps an optional field can be sent with the foreground event instead.