audioutilsprivate: restore thread priority before ending

The priority of the thread that executes audioringbuffer_thread_func
is incremented on Windows by the usage of the AvSetMmThreadCharacteristics
API. This change has to be restored, as described on the documentation
of the API (https://docs.microsoft.com/en-us/windows/win32/api/avrt/nf-avrt-avsetmmthreadcharacteristicsw#remarks),
with a call to the AvRevertMmThreadCharacteristics. If this is not done,
a handle will be leaked.

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