aout: dec: lock streams operations

Video decoders can already send data from any threads, this is now the case for audio decoders.

Indeed, nothing prevent from the documention and API to call vlc_aout_stream functions from any threads. This will be the case if we add an asynchronous audio decoder (we currently don't have any).

Some aout modules like Pulse and Android already have a lock between stream operations (so this new stream lock is superfluous).

Others aout modules like mmdevice/wasapi will need this new lock since stream operations are not guarded with a mutex.

Refs #27023 (closed)