Clarified lifecycle of ARSession along with ARCoreExtensions

[t5ujiri]

During development using the ARCore Unity Extension, inconvenience arises from not knowing when an ARCore Session is initialized and configured for authentication.

For example, in iOS, the API can be successfully called by activating the game object in XROrigin/ARSession/ARCoreExtensions that has been previously deactivated with the Start function and then setting SetAuthToken (however but there is a problem with the session not being set correctly immediately after the app is installed #76 ).
On the other hand, on Android, Keyless Authentication will not work correctly if the GameObject is disabled on the scene as described above.