If you are able to show test ads (which you are), then your implementation is good. I ran your project with your ad unit ids & got an error code 0 -- which means there are no ads available. It could be that there just aren't any ads being served to your app in my region (or for whatever reason), or there might be an issue on your AdMob account side. Make sure you have set up your payment details in your AdMob console.
Also, you might want to try adding your application ID to the manifest. According to the AdMob documentation, this is required from SDK 17.0.0 - but I'm not seeing an initialization crash, so maybe it's not an issue in your case.
AddApplicationText(
<meta-data android:name="com.google.android.gms.games.APP_ID"
android:value="[YOUR ADMOB APP ID]" />
)
Also have a search through the forums because there are a number of threads with the same issue you're having with a few different resolutions.
- Colin.