Make dependency on Error Prone Annotations non-optional

[Marcono1234]

Purpose

Avoid problems for users which could occur due to dependency being optional

Description

See discussion starting at #2320 (comment)
Making the dependency non-optional seems to be safer and avoids confusing warnings for users.

Checklist

• New code follows the Google Java Style Guide
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors

[eamonnmcmanus]

Thanks! As discussed previously, this does seem to be the right thing to do. As mentioned in that discussion, we might later decide to have Gson-private versions of the annotations we use (@Keep and @InlineMe), since their analysers only look at the simple name of the annotation. But the overhead of an annotation definition is pretty small so it's probably not worth doing that.