Bluetooth: Audio: Add fallback to get_chan_allocation

Added a fallback parameter to
bt_audio_codec_cfg_get_chan_allocation as absence of
channel allocation in BAP implicitly means Mono.
In the case that it is absent,
BT_AUDIO_LOCATION_MONO_AUDIO is the returned value.

This commit also fixes the implementation of
bt_audio_codec_cfg_get_frame_blocks_per_sdu as it only applies to
LC3 (as per the BAP spec). It also adds additional testing of it

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