Hygiene: Use mw.track to log events

Rather than loading the ReadingDepth Event Logging schema module and
logging an event ourselves, defer to the mechanism provided by EL
itself.

This decouples the component that generates the data for the event from
the component that (lazily) logs events.

Bug: T155639
Change-Id: Ic55227b670be28989fc1a228888521f50d09297a