Updated accessibility nodes not properly cached.

We have an accessibiliy node info cache in the process of each
accessibility service. Sometimes the cached node changes and
in this case we have evict from the cache the sub-trees rooted
at children that are no longer present. There was bug where if
none of the children is present anymore we did not evict their
sub-trees.

Also the verification logic was not stopping if a duplicate
node is seen potentially getting into an infinite loop.

bug:15729762

Change-Id: I7adf4f0a1bde07b385556d9acdded5059231f1f3