Bluetooth: audio: tests: Switch to one-time adv

This patch removes all uses of the adv auto-resume feature in the audio
bsim tests, and instead makes all adv starts explicit.

The auto-resume feature is planned for deprecation. And, explicit
starting of adv makes what happens in the test more explicit as well.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>