Audio: Change how the end of the audio stream is detected by the leaf audio...

Audio: Change how the end of the audio stream is detected by the leaf audio component.  This used by be done by looking for the first partial buffer.  That does not work with the in-place sub-sampling performed by the PCM decoder:  That always reduces the size of the buffer so that all buffers only partially filled by the time they get to the leaf.  Now, a flag is set in the audio buffer flags set to indicate the final buffer in the stream.