hal: Add independent backend support for a2dp

- With touch tone enabled audio mute issues
  are seen during softhandoff between SBC to
  APTX BT switch

- BT IPC library communication is happening
  for BT & combo device.
  Output session reconfigure to new encoder only
  on force device switch flag is set.However flag is
  reset in a2dp module on session closer.This results
  concurrent session to continue on previous encoder
  configuration which is observed as mute.

- Add independent backend support for speaker BT combo
  and communicate to a2dp module only on BT state change.
  Add new API in a2dp module to update force route flag.

Change-Id: Iffc34de62662b662387989af437dd11f94ec495f