le_audio: Introduce set/get BroadcastToUnicastFallbackGroup API

This CL extends the BluetoothLeAudio API to allow setting and getting a
fallback Broadcast to Unicast group.

Bug: 375421718
Bug: 375422410
Flag: com.android.bluetooth.flags.leaudio_broadcast_api_manage_primary_group
Test: atest BluetoothLeAudioTest
Change-Id: Ic0d64282b141c523b1813d2df1ae535bfbacf1c8