I just found this out recently and it's a lifesaver. My account never got linked to Google. I'd also had a trying time trying to move my game from the old device to the new device. This works on any device by the way.
- Open the account tool bar on the bottom. Go to Settings. It's that rusted gear.
- Hit account.
- You'll get a prompt to link your account to Facebook.
- Log into Facebook using the email and password you usually use for your facebook account.
- You are now linked.
The game whenever it gets installed on a new device just gives you a new account to play on. You hit that account button, pull up the login prompt and login again just as explained in step 4. Voila, the account is on the new device. Some people already do this on multiple accounts but there's probably alot of people in my situation who have reached level 24+ on a guest account and don't want to lose all that hard work.