LeAudio: Split devices.cc

Extract group related classes into a separate header and implementation
for better separation and readability.

Bug: 295972694
Test: atest --host bluetooth_le_audio_test bluetooth_le_audio_client_test bluetooth_test_broadcaster bluetooth_test_broadcaster_state_machine bluetooth_le_audio_codec_manager_test
Change-Id: I92a7569b6f3891306a88ed89d7006ce681b9990f