audio: Use composition instead of multiple inheritance

- AudioTrack is derived from BnDirectTrackClient along
  with RefBase to receive the notifications from
  AudioFlinger in tunnel/LPA mode. This is breaking
  binary compatibility for the clients which use native
  AudioTrack directly instead of Java based AudioTrack
  API
- Fix is to use composition here instead of inheritance

Change-Id: I9fd2c61cff5629924e3b08f63559679e0263bcfd