leaudio: Refactor unidirectional handling for VOICE ASSISTANT

Change the way unidirectional configuration is choosen when remote
device has no support for ASSISTANT context on a single direction.

This is needed for simialar change in GAME context.

Note: Some cleaning on duplicated json configuration has been done in
this CL as well.

Bug: 351974325
Bug: 332510824
Test: atest bluetooth_le_audio_client_test bluetooth_le_audio_test
Flag: com.android.bluetooth.flags.le_audio_support_unidirectional_voice_assistant
Change-Id: I3bda2a1b182fadac805e1a9a3614ce43587823cb