le_audio: Allow to set up other than MEDIA as default context

Patch allows to set up MEDIA as default context. This could be critical
for some kind of devices that does not supports MEDIA context.

Test: atest bluetooth_le_audio_client_test
Sponsor: @jpawlowski
Bug: 229802353
Change-Id: Ib8a59f974f9c1502068fc4ac29764829c37a34ea
Merged-In: Ib8a59f974f9c1502068fc4ac29764829c37a34ea