Monitor inactivity in the viewability AB test

Pauses the recording and logging of advert dwell time if we deem the user to be inactive.

Inactivity for the purpose of this test describes the absence of click/mouse/key/scroll events on the page for 5 seconds.
Removed a few bugs I introduced
Use awayTimeout variable
Merge pull request #2610 from guardian/idle-time

Monitor inactivity in the viewability AB test