Sets the current user that will be logged in when communicating with Gladly. If this is set the user does not have to provide email and name.
If a userSettings has previously been set, an Error will be thrown
context - Activity/application context to be initialized with.
user - The user settings that Gladly will use.
GladlyError.UserAlreadyDefined - if a user has previously been defined.
GladlyError.NotInitialized - if Gladly has not been initialized.