Implement MonoPipe::getTimestamp

using SingleStateQueue observer

Change-Id: I7b1928b087f1e676c7b291df6cefa7707301662c
diff --git a/media/libnbaio/MonoPipe.cpp b/media/libnbaio/MonoPipe.cpp
index b55be83..de0ad28 100644
--- a/media/libnbaio/MonoPipe.cpp
+++ b/media/libnbaio/MonoPipe.cpp
@@ -315,6 +315,9 @@
 
 status_t MonoPipe::getTimestamp(AudioTimestamp& timestamp)
 {
+    if (mTimestampObserver.poll(timestamp)) {
+        return OK;
+    }
     return INVALID_OPERATION;
 }