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/Texture.cpp

@@ -28,8 +28,8 @@
 #include <filament/Stream.h>
 #include <filament/Texture.h>
 
-#include "CallbackUtils.h"
-#include "NioUtils.h"
+#include "common/CallbackUtils.h"
+#include "common/NioUtils.h"
 
 using namespace filament;
 using namespace backend;