Audio: Make ABuffer sit on top of a SharedBuffer.

This allows us to carry the same buffer all the way from the WAV loader
to the AudioServer mixer.

This alleviates some of the stutter, but there's still a noticeable
skip when switching buffers. We're gonna need to do better. :^)

Libraries/LibAudio/AWavLoader.cpp

@@ -254,5 +254,5 @@ RefPtr<ABuffer> ABuffer::from_pcm_data(ByteBuffer& data, int num_channels, int b
     // don't belong.
     ASSERT(!stream.handle_read_failure());
 
-    return adopt(*new ABuffer(move(fdata)));
+    return ABuffer::create_with_samples(move(fdata));
 }