[media] Move audio_core_impl to using fidl instead of thread_set_priority

One notable change is that a 50ms delay is removed before publishing
services. As far as I can tell this doesn't seem to have any adverse
effect.

ZX-2914 #comment [media] Move audio_core_impl to using fidl instead of thread_set_priority

Test: CQ, manual on device
Change-Id: Ic914b022571c9e7a8f28ed65a784a5747aac584a