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.h b/media/libmediaplayer2/nuplayer2/NuPlayer2.h
index 798c725..b8fb988 100644
--- a/media/libmediaplayer2/nuplayer2/NuPlayer2.h
+++ b/media/libmediaplayer2/nuplayer2/NuPlayer2.h
@@ -197,6 +197,7 @@
     sp<DecoderBase> mVideoDecoder;
     bool mOffloadAudio;
     sp<DecoderBase> mAudioDecoder;
+    Mutex mDecoderLock;  // guard |mAudioDecoder| and |mVideoDecoder|.
     sp<CCDecoder> mCCDecoder;
     sp<Renderer> mRenderer;
     sp<ALooper> mRendererLooper;