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/RenderableManager.cpp

@@ -18,7 +18,7 @@
 #include <jni.h>
 
 #include <filament/RenderableManager.h>
-#include "NioUtils.h"
+#include "common/NioUtils.h"
 
 using namespace filament;
 using namespace utils;