Reset direct output thread timestamp on flush

In order to avoid subsequent tracks using stale kernel timestamps while
computing track timestamps, clear the timestamp info upon flush.

Change-Id: I2b8af33b287aee986871b6415b89f9c53d92f3a6

services/audioflinger/Threads.cpp

@@ -6013,6 +6013,7 @@ void AudioFlinger::DirectOutputThread::flushHw_l()
     mHwPaused = false;
     mFlushPending = false;
     mTimestampVerifier.discontinuity(); // DIRECT and OFFLOADED flush resets frame count.
+    mTimestamp.clear();
 }
 
 int64_t AudioFlinger::DirectOutputThread::computeWaitTimeNs_l() const {