message: Add `replaceGlobalInstanceTestOnly(...)` to MutablePrimitiveRegistry. That allows us to do testing for Registry and other classes which need control over what's registered in the .*PrimitiveRegistry, not impeding the ordinary usages in other classes like KeyTypeManagers.
  NOKEYCHECK=True
diff: @@ -33,11 +33,15 @@ import java.util.concurrent.atomic.AtomicReference;
  * should register such an object into a global, mutable registry.
  */
  public final class MutablePrimitiveRegistry {
  - private static final MutablePrimitiveRegistry GLOBAL_INSTANCE =
  + private static MutablePrimitiveRegistry globalInstance =
  new MutablePrimitiveRegistry();
  public static MutablePrimitiveRegistry globalInstance() {
  - return GLOBAL_INSTANCE;
  + return globalInstance;
  + }
  +
  + public static void resetGlobalInstanceTestOnly() {
  + globalInstance = new MutablePrimitiveRegistry();
  }
  private final AtomicReference<PrimitiveRegistry> registry =
  