Fixes #4544

Background: Dagger's generated factory has an optimization to lazy load a singleton instance of the factory when there are no dependencies:

```
public final class Foo_Factory implements Factory<Foo> {
  public static Foo_Factory create() {
    return InstanceHolder.INSTANCE;
  }

  private static final class InstanceHolder {
    private static final Foo_Factory INSTANCE = new Foo_Factory();
  }
}
```

The `private` modifier on the `INSTANCE` field is not needed and can add extra overhead as described in #4544. Removing the `private` modifier is also needed once we start generating Kotlin sources since accessing a `private` field on a nested class in kotlin source is a kotlinc error.

RELNOTES=Fixes #4544: Removes private from InstanceHolder field.
PiperOrigin-RevId: 708101532