audio: Starting proxy thread when there is an active stream

- Proxy thread is started from DoRouting only when previous
  and current devices are external devices. When CS call
  is ended while BT Headset is connected, we get a route
  call to ear piece intermittently and then for BT A2DP to
  resume the music playback. As previous device is not
  external device, Proxy thread is not started automatically
  in this scenario. This is blocking pcm_write call for
  touch tones as ear piece is disabled and data is not
  consumed by proxy device as well

- Fix is to start proxy thread after the route call when
  pcm_write for any stream is in progress

Change-Id: I378419977a2ac5af6cd86aa52bf0cc97a3abcee8
CRs-fixed: 661252