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/include/media/nbaio/MonoPipeReader.h b/include/media/nbaio/MonoPipeReader.h
index 0e1c992..78fe867 100644
--- a/include/media/nbaio/MonoPipeReader.h
+++ b/include/media/nbaio/MonoPipeReader.h
@@ -49,6 +49,8 @@
 
     virtual ssize_t read(void *buffer, size_t count, int64_t readPTS);
 
+    virtual void    onTimestamp(const AudioTimestamp& timestamp);
+
     // NBAIO_Source end
 
 #if 0   // until necessary