Remove private from generated Factory's INSTANCE field.

[copybara-service]

Remove private from generated Factory's INSTANCE field.

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.