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/media/libnbaio/MonoPipe.cpp b/media/libnbaio/MonoPipe.cpp
index e8d3d9b..a74b49e 100644
--- a/media/libnbaio/MonoPipe.cpp
+++ b/media/libnbaio/MonoPipe.cpp
@@ -310,4 +310,9 @@
     return mIsShutdown;
 }
 
+status_t MonoPipe::getTimestamp(AudioTimestamp& timestamp)
+{
+    return INVALID_OPERATION;
+}
+
 }   // namespace android