Make sure mFocusedStack is never the home stack.

There were circumstances where mFocusedStack could be assigned the
home stack. If this were ever to occur then all subsequent tasks would
be put on the home stack. This fix ensures that there is no way that
mFocusedStack will ever be assigned to the home task.

Fixes bug 11271189.

Change-Id: I7ddd9b6bcbf2787cbe2f44b461ad057ae2241f00