code: @SuppressWarnings("unused")
      public void addMentionWatcher(@NonNull MentionWatcher watcher) {
          if (!mMentionWatchers.contains(watcher)) {
              mMentionWatchers.add(watcher);
          }
      }
docs: Register a {@link com.linkedin.android.spyglass.ui.MentionsEditText.MentionWatcher} in order to receive callbacks
  when mentions are changed.
  
  @param watcher the {@link com.linkedin.android.spyglass.ui.MentionsEditText.MentionWatcher} to add