title: getRef(position).getKey() in RecyclerView.Adapter
body: <p>In FirebaseRecyclerAdapter I can use the <code>getRef(position).getKey()</code> but I am using a RecyclerView.Adapter and I want get the key whenever the user click the checkbox. Here is my code in onBindViewHolder():</p>
  
  <pre><code>    @Override
  public void onBindViewHolder(@NonNull UsersViewHolders holder, final int position) {
      holder.setName(mUserNameList.get(position));
      holder.setDesc(mUserDescList.get(position));
      holder.setImage(mUserPicList.get(position));
      holder.mCheckBox.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
  
          @Override
          public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
  
              //TODO: get the key
  
          }
      });
  }
  </code></pre>
  
  <p>Is it possible to get the key in RecyclerView.Adapter?</p>