Constructor for TwKeyChainHostApi. The binaryMessenger named argument is
available for dependency injection. If it is left null, the default
BinaryMessenger will be used which routes to the host platform.
Fetches a value from the keychain.
key is the key to fetch.
service is the service to fetch from.
accessGroup is the access group to fetch from.
Returns the value stored in the keychain.
Saves a value to the keychain.
key is the key to save.
value is the value to save.
service is the service to save to.
accessGroup is the access group to save to.