Accessibility should not query for list items when data set changed.

AbsListView is backed by an adapter. After the adapter data changes
the view sets a flag that its state is dirty and requests a layout.
If an accessibility service asks for the state of a list item at
this point, it may incur an error since the views and the adapter
are not in sync.

Now if an accessibility service queries for a list item when the
data set is changed and the item views are dirty, we pretend the
children do not exist. After the layout happens, we notify the
accessibility layer that the screen content changed so it can
refetch the views if desired (this notification mechanism is
already in place in AbsListView#handleDataChanged()).

bug:8433433

Change-Id: I4287a0ac2ef6bb33f1f988d5ddad973556c305ca