linux-pulseaudio: Use interpolated system timing

Before it was giving timestamps based upon system time for each new
segment of audio data.  Also, it was subtracting pulse latency from the
audio timestamp, which seems like it was really meant for use with the
pulse audio time rather than system time.

Now, it just uses system time for timestamps.  Still might not be
totally perfect, but seems to be much better than it was.

This also removes the latency calculation.  Latency is no longer used
because we're not using pulseaudio timing.