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
(cherry picked from commit 8fd102e618a3110bd5b8ba5849c57914445e2ef5)