audio: add linein to BT headset loopback support

- Add AUDIO_DEVICE_OUT_BLUETOOTH_A2DP as the sink
  device to support linein to BT headset loopback.
- set device_connect parameter in transcode loopback
  test app to load A2DP source IPC library.

Change-Id: Ib52fdf9aa966f50834ffa34884f5fa76053df15e
3 files changed