Improve concurrency access for updateMetrics()

nuplayer's updateMetrics() referenced an unprotected shared stats buffer.
It's a small buffer, so we now make a copy during updateMetrics()
[at a point where we are mutexed] instead of putting a mutex on the
underlying frequently used construct.

Ensure that nuplayer2 has the same protections.

Bug: 123256408
Test: race condition
diff --git a/media/libmediaplayer2/nuplayer2/NuPlayer2.cpp b/media/libmediaplayer2/nuplayer2/NuPlayer2.cpp
index 5da6e24..100cdae 100644
--- a/media/libmediaplayer2/nuplayer2/NuPlayer2.cpp
+++ b/media/libmediaplayer2/nuplayer2/NuPlayer2.cpp
@@ -1289,6 +1289,7 @@
             } else if (what == DecoderBase::kWhatShutdownCompleted) {
                 ALOGV("%s shutdown completed", audio ? "audio" : "video");
                 if (audio) {
+                    Mutex::Autolock autoLock(mDecoderLock);
                     mAudioDecoder.clear();
                     mAudioDecoderError = false;
                     ++mAudioDecoderGeneration;
@@ -1296,6 +1297,7 @@
                     CHECK_EQ((int)mFlushingAudio, (int)SHUTTING_DOWN_DECODER);
                     mFlushingAudio = SHUT_DOWN;
                 } else {
+                    Mutex::Autolock autoLock(mDecoderLock);
                     mVideoDecoder.clear();
                     mVideoDecoderError = false;
                     ++mVideoDecoderGeneration;
@@ -1967,6 +1969,7 @@
         int64_t currentPositionUs, bool forceNonOffload, bool needsToCreateAudioDecoder) {
     if (mAudioDecoder != NULL) {
         mAudioDecoder->pause();
+        Mutex::Autolock autoLock(mDecoderLock);
         mAudioDecoder.clear();
         mAudioDecoderError = false;
         ++mAudioDecoderGeneration;
@@ -2085,6 +2088,8 @@
         }
     }
 
+    Mutex::Autolock autoLock(mDecoderLock);
+
     if (audio) {
         sp<AMessage> notify = new AMessage(kWhatAudioNotify, this);
         ++mAudioDecoderGeneration;
@@ -2395,6 +2400,8 @@
     CHECK(mTrackStats != NULL);
 
     mTrackStats->clear();
+
+    Mutex::Autolock autoLock(mDecoderLock);
     if (mVideoDecoder != NULL) {
         mTrackStats->push_back(mVideoDecoder->getStats());
     }