Taking into account data change for AbsListView when prefetching node infos.

We are prefetching accessibility node infos to minimize the number of IPC
calls when an accessibility service introspects the screen. It is however,
possible that the view we are prefetching is a child of an AbsListView whose
adapter changed its data but the AbsListView still did not perform a layout
pass to sync its children with the new adapter state. This may lead to an
exeption when trying to query for the state of a child's position. If the
data of the adapter is changed and the layout pass still not performed,
we return null for the AbsLIstView's children. When the layout pass
completes we already notify the accessibliity layer so it will be able to
refetch the children of the AbsListView.

bug:8433433

Change-Id: I56313c721aef3848b15fad50027d068ba1d291f7