Switch the GoogleAdMobAdsSdk to Google Play Service SDK.

Because Google has announced that Google Mobile Ads SDK will be deprecated on August 1, 2014, and replace it with Google Play SDK. Refer to https://developers.google.com/mobile-ads-sdk/docs/admob/fundamentals#android.

So adjust the related code according to official suggestion (https://developers.google.com/mobile-ads-sdk/docs/admob/play-migration),