Android: move JNI C++ utils into a common folder.

No need for a proper library, this is just a common location to simplify
JNI bindings in other projects like filamat and gltfio.

There is no need to move the one Java source file (`NioUtils.java`)
since downstream libraries will have a dependency on Filament, and
FindClass should work fine.

android/filament-android/src/main/cpp/VertexBuffer.cpp

@@ -24,8 +24,8 @@
 #include <math/vec3.h>
 #include <math/vec4.h>
 
-#include "CallbackUtils.h"
-#include "NioUtils.h"
+#include "common/CallbackUtils.h"
+#include "common/NioUtils.h"
 
 using namespace filament;
 using namespace filament::math;