firebase-admin-java (6.4.0) does not yet expose the FirestoreOptions#setTimestampsInSnapshotsEnabled(boolean) method :
https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-clients/google-cloud-firestore/src/main/java/com/google/cloud/firestore/FirestoreOptions.java#L203

I would like to keep our POJOs similar between our Android code base and our Java code that uses Firebase Admin (for Firestore).

I have a pull request that is almost ready which will solve this. I'm just filing an issue as per the directions on the pull request template.