Clear the timestamp latch valid flag if no new timestamp available

Bug: 11815245
Change-Id: I94885a1b4c2c7c055b8eacc7f7d0fcd064094b6d
diff --git a/services/audioflinger/Tracks.cpp b/services/audioflinger/Tracks.cpp
index c5ab832..6482f68 100644
--- a/services/audioflinger/Tracks.cpp
+++ b/services/audioflinger/Tracks.cpp
@@ -853,6 +853,7 @@
         if (mState == FLUSHED) {
             mState = IDLE;
         }
+        mPreviousValid = false;
     }
 }