/platform/frameworks/base: Add support for A2DP notification to audioflinger

clients

- Whenever A2DP is connected, clients of audioflinger were not notified.
- This is needed incase of LPA decoding, to ensure that the A2DP is
  handled and stream is routed to correct outputs.
- Add support for A2DP notification when the client is registering (if A2DP
  is already connected).
- Add new API in audioflinger to de-register the client - for multiple
  playbacks, otherwise AF will have the old client and it will result in
  wrong notification.

Change-Id: I56a7a8e25af8313a616164f9ba286be979364261