Track document visibility in reading depth schema

When document becomes hidden/visible calculate milliseconds paused
and subtract this from totalLength upon logging pageUnloaded

If a user opens a link in a new tab and then tabs to it,
first paint will correspond to the time the tab was switched to.
We also have some special casing for the case where DomInteractive is
available but not first paint.

Bug: T155639
Change-Id: I762ec3fc91decf3cffa869dbd783faf62f01329a