Add support for Multi-HFP devices

1. Add multi headset support in stack.
2. Each HS will be associated with unique CB in BTIF
and also corresponding SCB in BTA.
3. Upstream event callbacks will be sent to
specific HS based on BD_ADDR.
4. Similiarly all commands to BTIF will be for
specific HS.

Change-Id: I0dc6b1b54f16244b802d818ae60944877a41d020
6 files changed