[Java] Improper escaping of keywords in field names 

When I created #7137 to address keywords escaping in typescript, I added the following stanza to `monster_test.fbs` to test things:

```
enum class: int {
  new,
  instanceof,
}

table Object {
  return:int;
  if:int;
  switch:int;
  enum:class;
}
```

This resulted in the following output for Java:
https://github.com/google/flatbuffers/runs/5382596719?check_suite_focus=true