libsysutils: SocketListener export release

* Foreground thread (legacy) closes connection by
  reporting false return value for onDataAvailable.
* Background thread can choose instead to close the
  connection asynchronously by calling release, must
  return true for the onDataAvailable method.

(cherry picked from commit d7ad4e40)

Change-Id: I70ab37d5c22b02804aa3b4dfb26cc9b75291f8b6