Floss: Add Three-way call support

When HFP audio path is connected (CRAS call) before an incoming call the
stack is in a multi-party state, while the headset has been informed that
this is not supported (Three-way flag in +BRSF event). In this case the
headset is only able to reject the incoming call.

Bug: 300409919
Test: Conduct the following manual tests.
  pair Bluetooth Headset
  load https://online-voice-recorder.com and start recording
  load https://google.git.io/libhidtelephoony/ with Chrome
  click on "Start", select the headset then "Connect"
  "Inputs Reports" and "Output Reports" are correct
  start "Hook Switch" test case
  no ringtone is heard in headset
  accept call from headset
  hang up call from headset
  "Hook Switch" test case is pass and recording audio continues
Test: atest bluetooth_test_gd
Change-Id: I9980ad57ec83696bb0df53a9ed861b52a1950246