Floss: implement le_audio_software_host

The audio HAL architecture diverges from that of Android (which uses
AIDL/HIDL), and |le_audio_software_host| serves as the symmetric
solution to manage that interface between the BT and the Audio stack.

Bug: 317682584
Test: m Bluetooth
Flag: EXEMPT floss only changes
Change-Id: Ifb9cfb3ce3ae30ebb51fb196e9524eff914dde13