AudioServer: Stop re-creating the device stream buffer

The buffer provided to `OutputMemoryStream` was made a private class
member. This is because there is no reason to re-create it in every
iteration. Also, the logic becomes more symmetric with
`m_zero_filled_buffer` which is already a class member.

Userland/Services/AudioServer/Mixer.h

@@ -133,6 +133,7 @@ private:
     NonnullRefPtr<Core::ConfigFile> m_config;
     RefPtr<Core::Timer> m_config_write_timer;
 
+    Array<u8, HARDWARE_BUFFER_SIZE_BYTES> m_stream_buffer;
     Array<u8, HARDWARE_BUFFER_SIZE_BYTES> const m_zero_filled_buffer {};
 
     void mix();