Fix setSettings not updating _settings in RNCallkeepModule due to singleton

[manuquentin]

In RNCallkeepModule.setSettings we called:

// ...
_settings = getSettings(null);

But in getSettings we're only updating _settings if its null:

public static WritableMap getSettings(@Nullable Context context) {
    if (_settings == null) {
        fetchStoredSettings(context);
    }

    return _settings;
}

So _settings is never updated we calling setSettings and will have the value of the first read in fetchStoredSettings.