audio: Initialized mAttributes in AudioTrack

- DirectAudioTrack creation fails for LPA/Tunnel playback
  if pAttributes passed to constructor is NULL. The reason is
  if pAttributes is NULL mAttributes is not initialized.
  getOutputForAttributes reads random usage value due to
  this reason
- Fix is to initialise mAttributes to default value.

CRs-Fixed: 811692
Change-Id: I0e747c6b46215a9b52e9d489c292b1046bd08f97