R1:7f5b15c6faaf

Made some classes final.

Summary:
Currently, the linter raises `XHP29` warnings for these files because they are not abstract or final.

I guess there are two possibly solutions, either making the classes final or marking them as `@concrete-extensible`. Given that there are no subclasses of these classes in the `phabricator`, `arcanist` and `libphutil` repositories... I opted to declare the classes as final.

Test Plan:
The following linter warnings are gone:

```
>>> Lint for…