le_audio: Implement default setter for fallback to unicast group

This CL add mechanism that sets earliest connectedi and assigned to
group LE Audio device as default group.

Bug: 375421718
Bug: 375422795
Flag: com.android.bluetooth.flags.leaudio_broadcast_primary_group_selection
Test: atest LeAudioServiceTest
Test: atest LeAudioBroadcastServiceTest
Change-Id: Ic38a97bdcd1cd57f3597526cf8798c5d6aaabce6