@NonNull
        public Builder setKey(@Nullable String key) {
            mKey = key;
            return this;
        }
Set a unique identifier for this {@link Person}. This is especially useful if the
{@link #setName(CharSequence)} value isn't unique. This value is preferred for
identification, but if it's not provided, the person's name will be used in its place.
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/compat/Person.java