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/AudioStreamOutSink.h b/include/media/nbaio/AudioStreamOutSink.h
index 5976b18..7948d40 100644
--- a/include/media/nbaio/AudioStreamOutSink.h
+++ b/include/media/nbaio/AudioStreamOutSink.h
@@ -52,6 +52,8 @@
     // implementation of GNWT (if any)
     virtual status_t getNextWriteTimestamp(int64_t *timestamp);
 
+    virtual status_t getTimestamp(AudioTimestamp& timestamp);
+
     // NBAIO_Sink end
 
 #if 0   // until necessary