public synchronized void add(C callback) {
        if (callback == null) {
            throw new IllegalArgumentException("callback cannot be null");
        }
        int index = mCallbacks.lastIndexOf(callback);
        if (index < 0 || isRemoved(index)) {
            mCallbacks.add(callback);
        }
    }
Add a callback to be notified. If the callback is already in the list, another won't
be added. This does not affect current notifications.

@param callback The callback to add.
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/util/obs/CallbackRegistry.java