code: @Nullable
    public synchronized List<ProducerContextCallbacks> setPriorityNoCallbacks(Priority priority) {
      if (priority == mPriority) {
        return null;
      }
      mPriority = priority;
      return new ArrayList<>(mCallbacks);
    }
docs: Changes priority.
  
  <p> This method does not call any callbacks. Instead, caller of this method is responsible for
  iterating over returned list and calling appropriate method on each callback object.
  {@see #callOnPriorityChanged}
  
  @return list of callbacks if the value actually changes, null otherwise