Add NBAIO_Sink::getTimestamp()

with a real implementation in AudioStreamOutSink
for dummy implementation initially in MonoPipe.

Use in AudioFlinger::PlaybackThread::threadLoop_write()
to keep the input to the timestamp latch up-to-date.

Change-Id: I10ef277991b63bb43d55d6f3df75116ef32246cd
diff --git a/include/media/nbaio/MonoPipe.h b/include/media/nbaio/MonoPipe.h
index 5fcfe9e..ffdcc21 100644
--- a/include/media/nbaio/MonoPipe.h
+++ b/include/media/nbaio/MonoPipe.h
@@ -88,6 +88,9 @@
             // Return true if the write side of a pipe is currently shutdown.
             bool    isShutdown();
 
+            // Return NO_ERROR if there is a timestamp available
+            status_t getTimestamp(AudioTimestamp& timestamp);
+
 private:
     // A pair of methods and a helper variable which allows the reader and the
     // writer to update and observe the values of mFront and mNextRdPTS in an