Improve multiple tour support:

* Store tour start time in cookie
* If multiple tours want to show on the page, show the most recent
* Continue refactoring process, adding a ext.guidedTour.lib.internal
  module.  This will allow have testable methods that are not part
  of the public API.  The migration of internal methods to there is
  in progress.

Bug: 44924
Change-Id: Iad9cccf292d0eadc83068d5ba1a6467a6897bedb
Merge "Improve multiple tour support:"