registerContentObserver method
Implementation
void registerContentObserver(Uri uri, ContentObserverCallback callback) {
log("callbacks uris ${_contentObserverCallbacks.keys}");
if (!_contentObserverCallbacks.containsKey(uri.toString())) {
_contentObserverCallbacks[uri.toString()] = [];
invokeMethod(_REGISTER_CONTENT_OBSERVER, {"uri": uri.toString()});
}
_contentObserverCallbacks[uri.toString()]!.add(callback);
}