Back out of Home settings when penultimate home app is uninstalled

When the user deletes the last-but-one, there's nothing more that
they can do in Home settings (and we'll be suppressing it from the
top level of the Settings UI as well), so we now back out with
a dialog explaining the situation.

Bug 10918801

Change-Id: Id03169dace976f7de639e3b3ab3d50ed161258d8