Add screen engagement tracking of time spent and list items scrolled on a screen

[matus-tomlein]

Add screen engagement tracking by adding a screen_summary context entity to the foreground, background, and a new screen_end event (tracked before a transition to the next screen view) with the following information:

1. time spent on the screen while the app was in foreground
2. time spent on the screen while the app was in background
3. number of list items viewed on the screen out of all items (as a replacement for scroll depth tracking)

In addition, we should provide a SwiftUI view modifier for tracking the list items viewed.