Update MAVLinkPayload.java

Hello, 
Never used git before, but I was using a local copy of your code and noticed it wasn't wasn't taking care of signed extension re: going from uint64_t -> Long (signed in java) produces all bits set.

Need to type cast long and int on masks otherwise you'll get signed extension in java. I believe the byte order should also be swapped but I haven't had time to confirm for sure.