The only component I have seen so far for Delphi XE5 which implements an Admob wrapper for Firemonkey is this one from Flash AV Software Corp. It looks like they have a custom classes.dex file which makes sense as you’d want to import Admob into the Android SDK wrapper app (there is an Android shim that launches the Delphi XE5 library where you app lives). The Admob component for Delphi Firemonkey Android is a commercial component. They also have a separate component for IOS. Don’t forget to include the Admob permissions in your Android Manifest file:
Admob for Delphi Firemonkey On Android by Delphi XE5 XE6 XE7 XE8 10 Seattle Berlin Tokyo Firemonkey, Delphi Android, Delphi IOS is licensed under a Creative Commons Attribution 4.0 International License.