Disable advertising id collection
According to the Firebase docs you can disable advertising id collection by setting:
<meta-data android:name="google_analytics_adid_collection_enabled" android:value="false" />
in your AndroidManifest.xml
under the <Application>
tag.
EDIT: It seems like people are having mixed success with this approach. Try adding
configurations { all*.exclude group: 'com.google.firebase', module: 'firebase-core' all*.exclude group: 'com.google.firebase', module: 'firebase-iid' }
to the Gradle app dependencies area as per comment below.