LeAudio: Encapsulate CIG logic in LeAudioDeviceGroup

A collection of helper functions for managing CISes were put
into a CIG related subclass to improve encapsulation and readability.
Unused code was removed.

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: I5855e96c862b3513720d0594d957997cafd34565