Access Token discovery fix

After using `setExtendedAccessToken()`, the new long expiry access token is stored in persistent data so `getAccessToken()` must look in the persistent data first before falling back to application access token. 

Putting priority to `getUserAccessToken()` in `getAccessToken()` causes nesting when only signed_request is supplied. To prevent `_oauthRequest()` from invoking `getAccessToken()` again, an access token must be set. In this case, application access token is used.