code: public void setCustomerDefaultSource(
              @NonNull String sourceId,
              @NonNull @Source.SourceType String sourceType,
              @Nullable CustomerRetrievalListener listener) {
          final Map<String, Object> arguments = new HashMap<>();
          arguments.put(KEY_SOURCE, sourceId);
          arguments.put(KEY_SOURCE_TYPE, sourceType);
  
          final String operationId = UUID.randomUUID().toString();
          if (listener != null) {
              mCustomerRetrievalListeners.put(operationId, listener);
          }
          mEphemeralKeyManager.retrieveEphemeralKey(operationId, ACTION_SET_DEFAULT_SOURCE,
                  arguments);
      }
docs: Set the default source of the current customer object.
  
  @param sourceId the ID of the source to be set
  @param listener a {@link CustomerRetrievalListener} to be notified about an update to the
  customer