Fix the buggy task visibility

TaskCofigFactory is looking at the wrong methods to define
if a ref is visibile to user. Fix to call RefControl.isVisible()
rather than RefControl.canRead(). canRead() only returns the
project permissions but isVisible() takes the user and ref
into account. Also modify a supporting test.

Originally-Authored-By: Adithya Chakilam
Change-Id: I89ff399e66f06133212825a0fb3f1239734fb1a9