Optimize JNI to managed byte array conversion in JavaUtil

Unity provides a helper method to convert a Java array into a managed
array which is much quicker than making repeated overhead-heavy
AndroidJavaClass CallStatic calls.
Added null check for the raw object returned from GetRawObject().

If a valid raw object containing a null value is passed to
AndroidJNIHelper.ConvertFromJNIArray, it will crash.
Merge pull request #110 from seandanger/master

Optimize JNI to managed byte array conversion in JavaUtil