Narrow down an excessive visibility of `AbstractBackend` members

[yevhenii-nadtochii]

I've extended visibility of the class constructor and the factory method for forcing loggers to protected.

These are needed in tests to create stub implementations. Having them package-private forces to declare stubs right in spec files, which is inconvenient if that stubs are pretty big.

When migrating this class to Kotlin, these methods should become internal because real implementations should not see these methods as open.