audio: Fix after aosp/2908743 (exit command handling)

In aosp/2908743, the intended behavior of the "halReservedExit"
command (stream exit) was inadventedly changed. Instead of
exiting from the thread's loop only when the cookie has
the expected value, it was actually exiting when the cookie
value is any but zero. The intended behavior is as follows:

 - the cookie has expected value: do not send reply, exit loop;
 - the cookie has unexpected but non-zero value: ignore,
     do not send reply (that's the point of using the cookie);
 - the cookie has a zero value: ignore, send a reply (this is
     for VTS testing).

Bug: 300181540
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I9945eb0ba9042993adac8599b18f241c4f69ca7a