Another false positive SING_SINGLETON_IMPLEMENTS_SERIALIZABLE when class is not a singleton (e.g. protobuf)

[cpfeiffer]

https://github.com/spotbugs/spotbugs/blob/master/spotbugs/src/main/java/edu/umd/cs/findbugs/detect/MultipleInstantiationsOfSingletons.java#L218 lists the criteria for recognizing a class as a Singleton. While already quite sophisticated, it wrongly recognizes some non-singleton classes as singleton. E.g. protobuf-generated classes that contain a builder to create an instance like this (GeneratedMessageV3 is Serializable).

Here's a manually simplified version of such a class generated by the protoc compiler:

public final class Person extends
    com.google.protobuf.GeneratedMessageV3 implements PersonOrBuilder {
  private static final long serialVersionUID = 0L;

  private static final Person DEFAULT_INSTANCE;
  static {
    DEFAULT_INSTANCE = new Person();
  }

  public static Person getDefaultInstance() {
    return DEFAULT_INSTANCE;
  }

  private Person(com.google.protobuf.GeneratedMessageV3.Builder<?> builder) {
    super(builder);
  }
  private Person() {
    name_ = com.google.protobuf.LazyStringArrayList.emptyList();
  }

  @java.lang.Override
  @SuppressWarnings({"unused"})
  protected java.lang.Object newInstance( <--- this method should already be an indicator that it is not a singleton
      UnusedPrivateParameter unused) {
    return new Person();
  }

  public static Builder newBuilder() {
    return DEFAULT_INSTANCE.toBuilder();
  }

  public static final class Builder extends
      com.google.protobuf.GeneratedMessageV3.Builder<Builder> implements PersonOrBuilder {
    public Person build() {
      return new Person(this);
    }
  }
}

[cpfeiffer]

(Related to #2964)