Add missing header in android_media_MediaCodecList.cpp

Skia headers have changed and through some transitive dependencies
cause Android to not compile because this file does not include
some headers that it uses.

Test: builds
Change-Id: Ied8c2c6c456e9503553b22ff408148d5074f1404

media/jni/android_media_MediaCodecList.cpp

@@ -26,6 +26,7 @@
 
 #include <utils/Vector.h>
 
+#include <mutex>
 #include <vector>
 
 #include "android_runtime/AndroidRuntime.h"