Fix use of SkMutex.

Recent change to Skia moved SkMutex into its own header file. Include
it to use and SkMutex.

Use SK_DECLARE_STATIC_MUTEX to remove static initializer/finalizer.

Fixes master-skia build.

Change-Id: I2ab43c511587167b81310d5d55a65604d82761f5

core/jni/android/graphics/AutoDecodeCancel.cpp

@@ -1,6 +1,7 @@
 #include "AutoDecodeCancel.h"
+#include "SkMutex.h"
 
-static SkMutex  gAutoDecoderCancelMutex;
+SK_DECLARE_STATIC_MUTEX(gAutoDecoderCancelMutex);
 static AutoDecoderCancel* gAutoDecoderCancel;
 #ifdef SK_DEBUG
 static int gAutoDecoderCancelCount;