hal: Avoid A2dp route updation for delayed a2dp/hfp disconnect

- Existing code routes to speaker until stream goes to
  standby on A2dp disconnect. This is done to not block
  writes from audioflinger.
- Consider a dual a2dp/hfp scenario with A2dp connected
  and hfp being constantly reconnected.
- In failure case, routing=0 from hfp disconnect comes in
  late i.e. during hfp reconnect. Stream is resumed and
  until hfp connect is processed, it is routed to speaker.
- Fix this by not routing to speaker if A2dp has not
  disconnected by checking lower stack.

Change-Id: Idd6c827a391777e943b3d59f33f8f4e98f98e802