Support entity classes that are inner classes

Describe the bug
Placing entity classes annotated with @Entity and @BaseEntity in another class will result in an error when building.

Basic info (please complete the following information):

• ObjectBox version: 2.9.1
• Reproducibility: always
• Device: N/A (compile time error)
• OS: N/A (Android library)

To Reproduce
Steps to reproduce the behavior:
1. Create the following hierarchy of classes and build

public class Holder {
    @BaseEntity
    public static abstract class Parent {
        @Id
        long boxId;
        private String text;

        public String getText() {
            return text;
        }

        public void setText(String text) {
            this.text = text;
        }
    }

    @Entity
    public static class Child extends Parent {
    }
}

Logs, stack traces
Won't provide because this won't help in any way as IDE's build errors will be much clearer when dev-team build it themselves

The report names no source files, tests, or entry points. Start by locating the entity annotation and build-processing path for the shown Holder.Parent/Child hierarchy, then verify that building this example succeeds without the reported error.