audio: Mitigate double receiving of the "exit" command

In rare cases, a worker thread from a new stream created
via StreamSwitcher can read again from the command FMQ
the "exit" command which was sent to the worker of
the previous stream.

The underlying reason for that has to be investigated.
For now, mitigate the issue by salting the cookie of
the "exit" command with the worker's TID.

Bug: 300130515
Test: atest VtsHalAudioCoreTargetTest
Change-Id: Ie7d2e847e8b39414ffd31afd64e32d4c9a292c03