Improve way gt.currentTour is handled and deprecate gt.initGuider

* After gt.currentTour changed from string to object, it basically
happened to work since "any string".shouldLog is falsy.
* Put gt.currentTour back to old-style API (a tour name written by a
tour).
* Use new, private, structure currentTourState (recently
gt.currentTour).  Populate this in gt.initGuider as well.
* Deprecate gt.initGuider.  It harms maintainability, and the port to
defineTour is currently easy.

Change-Id: I9fe34a810cd80f3f2af999767e9930c5856a4243