Add NBAIO_Source::onTimestamp()

with dummy default implementation, and implement in MonoPipeReader.
onTimestamp is meant to be called by the corresponding sink when it has
a new timestamp available.

Change-Id: I8a90d24d1061e4a592ce5bd8ee1c9fce6bdd8a84
diff --git a/media/libnbaio/MonoPipeReader.cpp b/media/libnbaio/MonoPipeReader.cpp
index 394f6ac..851341a 100644
--- a/media/libnbaio/MonoPipeReader.cpp
+++ b/media/libnbaio/MonoPipeReader.cpp
@@ -86,4 +86,9 @@
     return red;
 }
 
+void MonoPipeReader::onTimestamp(const AudioTimestamp& timestamp)
+{
+    mPipe->mTimestampMutator.push(timestamp);
+}
+
 }   // namespace android