I've recently read about rewarded ads, as shown here:
I can't find any API about the recent rewards, of what the user got so far.
I tried to check on the docs and tried to check all possible functions of the API via the IDE itself.
The samples on Github show how to get a callback that you got a reward (so you can save it for later), but this will be lost in the case of uninstall of the app, or if you switch to another device that has the same app with the same user account.
AdMob does not keep a track of your users earning. You will have to keep the track of the users earning by yourself or by using some other service.
They will only provide you with an Video ad and will notify you once the Video a is completed so that you can award your users the points/virtual currency.
However there are other services such as Tapjoy which provide ads as well as provide you with the option to keep the track of your users earning. I think Tapjoy is free to use only if you use their Ad Network to show their ads to the user, which will help you to earn and also keep a track of the earnings under one portal.