Further improvement to determining visiblility of tasks in home stack.

673cbd2b introduced logic to make
non-top activities in the home stack invisible under certain
conditions. However, this caused problems with the set-up wizard which
uses the home stack to launch all its activties. Changed the logic to
determine if the next task in the home stack should be visible behind
vs. the next activity in the same task. So, activities in the same task
in the home stack can be visible at the same time, but not activities
in different tasks.

Bug: 26922407
Bug: 26571156
Change-Id: Ied20d45cd27a1adcc105703d8ca21861d1856700