Fix issue #3191573: PreferenceFragment.onActivityResult cannot

launch a new fragment on the same call.

There were some problems with the API design where you could do
things in such a way that a back stack entry that was not at the
top would get popped.  Ouch.  Hopefully this change prevents that
from being able to happen.

Change-Id: I8cbc952e12ddd231ff6c84b6e9bbf5125f449f04