You need to integrate Google Play Service in order to use Google Advertising ID with Bridge SDK.
*Necessary to use Google Advertising ID for Apps in Google Play Store
Here is the introduction to integrate Google Play Service:
■Android
1. Add the following to "build.gradle" > dependencies
compile 'com.google.android.gms:play-services-analytics:9.2.1'
2. Open Package Explore > Android Project > AndroidManifest.xml and add the following meta-data tag to <application> element
<meta-data android:name="com.google.android.gms.version" android:value="@integer/google_play_services_version" />
*Skip this step if you use Google Play Service version 7 or later
■Unity
1. Copy "google-play-services_lib" folder to Unity Project > Assets/Plugins/Android folder
2. Build App