public synchronized void clear() {
        if (mNotificationLevel == 0) {
            mCallbacks.clear();
        } else if (!mCallbacks.isEmpty()) {
            for (int i = mCallbacks.size() - 1; i >= 0; i--) {
                setRemovalBit(i);
            }
        }
    }
Removes all callbacks from the list.
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/util/obs/CallbackRegistry.java