title: What is "session_engaged" event property of "session_start" in Firebase?
body: <p>I'm working with Firebase analytics and I'm trying to figure out the best way to track the engagement of our (iOS app) users. 
  I see that there are multiple events and properties related to engagement, such as the "user_engagement" event (and the "engagement_time_msec" property) and "ga_session".
  User engagement seems a bit too low-level/frequent. It basically happens any time the user touches the screen and after booting the app I see that it is often sent 2/3 times ever before interacting with the app itself.
  While trying to find a better solution I stumbled across another event property, which is called "session_start" and <em>sometimes</em> it comes with a property called "session_engaged". It appears to be a bool or a counter (it's always 1 as far as I saw).
  I could not find any documentation on this event and property.</p>
  
  <p>Anybody who can give me more info on that?
  Thanks!</p>