frameworks: av: AudioPolicyEffect: Add refCount for each session

Audio framework stops and restarts AudioTracks on same audio session.
Sometimes the AudioTrack start() comes before a stop(). As effects
are linked to the session, they should only be created on the last
stop() related to that session.

Bug: 17201117

Change-Id: I19343e293d6b7b6b794fe05ebd0dd239ed3b075c