code: public static final Object registerInstance(Collator collator, ULocale locale) {
          return getShim().registerInstance(collator, locale);
      }
docs: <strong>[icu]</strong> Registers a collator as the default collator for the provided locale.  The
  collator should not be modified after it is registered.
  
  <p>Because ICU may choose to cache Collator objects internally, this must
  be called at application startup, prior to any calls to
  Collator.getInstance to avoid undefined behavior.
  
  @param collator the collator to register
  @param locale the locale for which this is the default collator
  @return an object that can be used to unregister the registered collator.
  
  @hide unsupported on Android