Review the public API and currently unused types

[armiol]

After splitting base into several repositories, some of the types and some of the methods became unused. Also, many endpoints are over-exposed — typically, being public instead of package-private.

During the migration to Java 11, the "unused" warnings were suppressed with

@SuppressWarnings("unused") /* Part of the public API. */

The over-exposed methods were left intact.

When addressing this issue, one should review the "unused" and over-exposed pieces. And either remove/hide them, or cover with the corresponding tests.