audiolayer: prevent double-init

src/media/audio/alsa/alsalayer.cpp

src/media/audio/audiolayer.cpp

src/media/audio/audiolayer.h

src/media/audio/coreaudio/corelayer.cpp

src/media/audio/coreaudio/corelayer.h

src/media/audio/jack/jacklayer.cpp

src/media/audio/jack/jacklayer.h

src/media/audio/opensl/opensllayer.cpp

src/media/audio/portaudio/portaudiolayer.cpp

src/media/audio/portaudio/portaudiolayer.h

src/media/audio/pulseaudio/pulselayer.cpp