ErrorProne: activate experimental checks

[0pdd]

The puzzle 1170-4b91f3a7 from #1170 has to be resolved:

mystamps/pom.xml

Line 799 in b78ce5b

<!-- @todo #1170 ErrorProne: activate experimental checks -->

The puzzle was created by @php-coder on 20-Jan-20.

[php-coder]

- AlmostJavadoc: disabled because of false positives: https://github.com/google/error-prone/issues/1430
- PrivateConstructorForUtilityClass: in our project the chances of this are rare and I don't want to add more code to suppress that
- AnnotationPosition: disabled because of false positives: https://github.com/google/error-prone/issues/1327

[php-coder]

Flag is -XepAllDisabledChecksAsWarnings

[php-coder]

More bug patterns: https://error-prone.picnic.tech