LeAudio: Decouple helper functions

This extracts, move or strips the argument lists of certain functions
from the redundant parameters in order to reduce the dependencies,
and to reduce the already inflated device and group class interfaces
at least a little bit. It will also allow us later to use the extracted
utility functions outside these classes (e.g. by the Codec Manager).

Bug: 308428860
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
Flag: EXEMPT; mechanical refactor; no functional change, confirmed with unit tests
Change-Id: I1482180ea2fd8a8f96f0047e1624b50bb108321f