audio: Fix handling of a thread exit command with a bad cookie

In case when the command was sent by the HAL itself (from another
thread), the worker thread must not post a reply. The only case
when a reply needs to be posted is in the case when the command
was sent from a VTS test. This case is identified by the fact that
the cookie has value '0'.

Bug: 300181540
Test: atest VtsHalAudioCoreTargetTest
Change-Id: Ifeb0722b5cf7346a694c5a938f6b324f5fa825f1