Fixing issues with the AccessibilityNodeInfo cache.

1. Before there were two caches one in the app process that
   kept track only the ids of infos that were given to a
   querying client and one in the querying client that
   holds the infos. This design requires precise sync
   between the caches. Doing that is somehow complicated
   since the app has cache for each window and it has
   to intercept all accessibility events from that window
   to manage the cache. Each app has to have a cache for
   each querying client. This approach would guarantee that
   no infos are fetched twice but due to its stateful nature
   and the two caches is tricky to implement and adds
   unnecessary complexity. Now there is only one cache in
   the client and the apps are stateless. The client is
   passing flags to the app that are a clue what nodes to
   prefetch. This approach may occasionally fetch a node
   twice but it is considerably simpler and stateless
   from the app perspective - there is only one cache.
   Fetching a node more than once does not cause much
   overhead compared to the IPC.

Change-Id: Ia02f6fe4f82cff9a9c2e21f4a36747de0f414c6f