Merge "Prevent adding duplicate callbacks to the state controller"

    @Override

    public void addCallback(@NonNull Callback callback) {

        Preconditions.checkNotNull(callback, "Callback must not be null. b/128895449");

        if (!mCallbacks.contains(callback)) {

            mCallbacks.add(callback);

        }

        if (mCallbacks.size() != 0 && !mListening) {

            mListening = true;

            mKeyguardUpdateMonitor.registerCallback(this);