fix(tracker): send unload event on page hidden instead

[ayuhito]

This removes all the extra logic used to calculate how long the user spent with the page hidden from view, and then calculating the full length spent subtracting time hidden. In theory, this seemed like a great idea, but in practice there were too many edge cases where the browser would send a ridiculously large time spent on page value that isn't representative of the user.

Now the tracker sends the unload event on the first time the page is hidden and considers that time spent on page. This should reduce the number of anomalies in data and not skew lesser visited pages as much.