Consider changing OccupancyGrid to use probabilities on 0-255 instead of 0-100

This is a follwup to the [pre Foxy Message API review](https://github.com/ros2/common_interfaces/pull/86) 

> Comment: probability being 0-100 doesn't take advantage of all the bits the `int8` type offers. 0-255 where 255 is 100% probability would have slightly better resolution.
