Bluetooth: Audio: Add fallback to cfg_meta_get_pref_context

Added a fallback parameter to
bt_audio_codec_cfg_meta_get_pref_context
as absence of pref context in BAP implicitly means a unspecified
for LC3.

In the case that it is absent BT_AUDIO_CONTEXT_TYPE_UNSPECIFIED
is the returned value.

While the metadata for codec cfg and codec cap are similar,
this only applies for codec cfg as per BAP.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>