Add schemas for screen engagement tracking on mobile

[matus-tomlein]

This PR adds 3 schemas for screen engagement tracking on mobile (screen_time and screen_end) and both on mobile and Web (view_segment).

These schema support the proposal presented in this demo.

• com.snowplowanalytics.snowplow/screen_summary/jsonschema/1-0-0
  • schema for entity tracked with mobile events (foreground/background/scren_end) with duration of the screen in foreground and background
• com.snowplowanalytics.snowplow/screen_end/jsonschema/1-0-0
  • schema for event tracked before transitioning to the next screen on mobile. it doesn't have any properties but will be tracked with the screen_time and view_segment entities

[matus-tomlein]

As I was implementing this in the iOS tracker, I iterated a bit on the schemas. I recorded a short video to demo how the schemas will be used in tracking, it can be found here.

Overall the changes are:

1. Renamed the vendor for all schemas to com.snowplowanalytics.mobile. All of these schemas are for mobile trackers and this is also the vendor used for the screen view event which they are used with.
2. In the screen_summary entity schema, I renamed the list item properties to be: last_item_index and items_count. So they will contain the index of the last viewed item and the number of all items in the list.
3. I added a third event schema for list_item_view event. If screen engagement is enabled on mobile, this event will be aggregated into the screen_summary context entity. It is just used as a way for users to update the currently seen list items.

[matus-tomlein]

Final update: after some discussion, we also added scroll view tracking in pixels (in addition to the list view tracking). We can't provide automatic tracking for this, but users can listen for scroll changed events in their scroll views and track a scroll_changed event. This is handled the same as the list_item_view event and aggregated in the screen_summary entity – there are two new properties: max_y_offset and content_height.

[rlh1994]

there are two new properties: max_y_offset and content_height

Can mobile applications have an x offset and width greater than the screen?

[matus-tomlein]

It'd be quite rare, but actually why not add it since we also have the horizontal scroll depth on Web too... Have updated the schemas to make it possible to track too.

[matus-tomlein]

One more update: have also added min_x_offset and min_y_offset to match with scroll depth tracking on the Web.

[matus-tomlein]

Cherry picked to R152