Floss: Disable three way call feature

To accurately represent Floss's capabilities during Bluetooth qualification, disable the three-way calling feature because it is not yet available.

Bug: 304919932
Test: atest bluetooth_test_gd
Tag: #floss
Flag: EXEMPT, no behavior change on Android; Floss-only changes

Change-Id: If04043e8ced828517b191e8ed4d634aeb18d4365