I am using the google play ads dependency just to get the advertising id.
com.google.android.gms:play-services-ads:7.8.0
Is there any way to avoid this as the library has a method count of 6201.
AdvertisingIdClient and its associated classes are also found in the much smaller
com.google.android.gms:play-services-basement:8.3.0
if you use Google Play services 8.3. Prior to that point, you are correct in that it was part of play-services-ads
.
In any case, it is strongly recommended that you use ProGuard to remove unused code from your app before shipping retail versions of your app.