Moved persistent key validation to BaseFacebook

When you extend the `BaseFacebook` class with your own persistent data handling methods, you're forced to handle session key validation on your own. And in order to write your own `clearAllPersistentData()` you have to have a full list of supported keys.

This functionality should be offered at the base class out-of-the-box so that the custom class can handle setting & unsetting persistent data without also having to worrying about session key validation.

This is also helpful for future cases when there might be another key needed by the SDK. Updates can be made to the base class and the validation will cascade.