Using wrong virtual view id When prefetching accessibility nodes.

When getting an accessibility node info we also prefetch its predecessors,
siblings and descendants up to fifty. If a view has a node provider, i.e.
virtual descendants, and the id of the virtual descendant is not specified
we use a speacial constant to refer that the provider should create a node
for its hosting view. The bug was that we were not using the correct constant
when calling into the provider.

bug:17664111

Change-Id: I4f6b2333e1e7e3e2bb6b164c0752613417c60d16