Merge changes I93166347,Id09322c5,I878f80f0,I7a7b1b44,Iae2dbc23

* changes:
  Expose new ContextHubService APIs
  Adds ContextHubClient to callback parameters
  Renames RESULT_FAILED_PENDING to RESULT_FAILED_BUSY
  Changes transaction Listener to OnCompleteListener
  Adds Nullable/NonNull annotations to Context Hub APIs