floss: notify hfp/a2dp audio status via forwarded fd

This adds a parameter to |start/stop_x| so that the audio server can
poll on the file descriptor for status update for faster transition.

Bug: 343885745
Flag: EXEMPT floss-only
Test: m Bluetooth

Change-Id: Iac311e1b96bca68bb0a2533c80cc6120bfda3ea5