Move EmailNotification watchlist handling into helper method

Pulls one pure method with no state from EmailNotification into a 
public static method.  This is done so that extensions (Flow, maybe
others) can replace the Email notifications with alternative ones (Echo)
while still updating the watchlist.

A better solution might be to extricate watchlist update from email
notifications, but this seems like a reasonable first step.

Bug: 66876
Change-Id: Iae213b87706c447b880244711e7747954423bb69
Merge "Move EmailNotification watchlist handling into helper method"