Refine AbsListView transcript mode behavior.

Make transcript mode only scroll to the bottom if the last item is
already scrolled fully to the bottom when the change occurs Before
this, transcript mode would cause a list to scroll to the bottom as
long as the last item in the adapter was visible at all. Long items
would make this behave unexpectedly.

Change-Id: Ia6a52c969ea6d50653cf2b8cd4dc30ba517cf803