audio: Start proxy when external output is enabled

- Proxy thread is not getting started when BT Headset
  is connected in the middle of HDMI Music Playback
- startPlaybackOnExtOut_l is not getting called
  from doRouting in this case
- Fix is to call startPlaybackOnExtOut_l in all the
  usecases when the device is switched to external
  device

Change-Id: Iabbe63bc87db0d0cc9623d16a24825118efacd29
CRs-fixed: 520503