Proguard/r8 instructions missing

From the Readme, I am following [Setup Instructions](https://googleapis.github.io/google-http-java-client/setup.html), then [Android](https://googleapis.github.io/google-http-java-client/android.html). Here it says "pay special attention to the Android instructions for [ProGuard](https://googleapis.github.io/google-http-java-client/setup.html#proguard)." but the referenced Proguard section does not exist.
I have been using this library with R8 successfully in the past, but recently tried out full mode, and got following exception:

```
Non-fatal Exception: java.lang.ExceptionInInitializerError:
at java.lang.Class.newInstance(Class.java)
at com.google.api.client.util.Types.newInstance(Types.java:113)
at com.google.api.client.util.Data.newMapInstance(Data.java:550)
at com.google.api.client.json.JsonParser.parseValue(JsonParser.java:766)
at com.google.api.client.json.JsonParser.parse(JsonParser.java:360)
at com.google.api.client.json.JsonParser.parse(JsonParser.java:335)
at com.google.api.client.json.JsonObjectParser.parseAndClose(JsonObjectParser.java:79)
at com.google.api.client.json.JsonObjectParser.parseAndClose(JsonObjectParser.java:73)
at com.google.api.client.http.HttpResponse.parseAs(HttpResponse.java:460)
at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.execute(AbstractGoogleClientRequest.java:603)
at org.totschnig.drive.sync.DriveServiceHelper.getFile(DriveServiceHelper.java:143)
at org.totschnig.drive.sync.GoogleDriveBackendProvider.requireBaseFolder(GoogleDriveBackendProvider.kt:403)
at org.totschnig.drive.sync.GoogleDriveBackendProvider.(GoogleDriveBackendProvider.kt:420)
at org.totschnig.drive.sync.GoogleDriveBackendProviderFactory.fromAccount(GoogleDriveBackendProviderFactory.kt:19)
at org.totschnig.drive.sync.GoogleDriveBackendProviderFactory.fromAccount(GoogleDriveBackendProviderFactory.kt:12)
at org.totschnig.myexpenses.sync.SyncBackendProviderFactory.from-0E7RQCE(SyncBackendProviderFactory.kt:18)
at org.totschnig.myexpenses.sync.SyncBackendProviderFactory.access$from-0E7RQCE(SyncBackendProviderFactory.kt:12)
at org.totschnig.myexpenses.sync.SyncBackendProviderFactory$Companion.get-0E7RQCE(SyncBackendProviderFactory.kt:41)
at org.totschnig.myexpenses.sync.SyncAdapter.onPerformSync(SyncAdapter.kt:123)
at android.content.AbstractThreadedSyncAdapter$SyncThread.run(AbstractThreadedSyncAdapter.java:354)

Caused by java.lang.IllegalArgumentException: unable to create new instance of class kc.a because it is abstract and because it has no accessible default constructor
at com.google.api.client.util.Types.handleExceptionForNewInstance(Types.java:162)
at com.google.api.client.util.Types.newInstance(Types.java:117)
at com.google.api.client.util.Data.createNullInstance(Data.java:166)
at com.google.api.client.util.Data.nullOf(Data.java:134)
at com.google.api.services.drive.model.File.(File.java:69)
at java.lang.Class.newInstance(Class.java)
at com.google.api.client.util.Types.newInstance(Types.java:113)
at com.google.api.client.util.Data.newMapInstance(Data.java:550)
at com.google.api.client.json.JsonParser.parseValue(JsonParser.java:766)
at com.google.api.client.json.JsonParser.parse(JsonParser.java:360)
at com.google.api.client.json.JsonParser.parse(JsonParser.java:335)
at com.google.api.client.json.JsonObjectParser.parseAndClose(JsonObjectParser.java:79)
at com.google.api.client.json.JsonObjectParser.parseAndClose(JsonObjectParser.java:73)
at com.google.api.client.http.HttpResponse.parseAs(HttpResponse.java:460)
at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.execute(AbstractGoogleClientRequest.java:603)
at org.totschnig.drive.sync.DriveServiceHelper.getFile(DriveServiceHelper.java:143)
at org.totschnig.drive.sync.GoogleDriveBackendProvider.requireBaseFolder(GoogleDriveBackendProvider.kt:403)
at org.totschnig.drive.sync.GoogleDriveBackendProvider.(GoogleDriveBackendProvider.kt:420)
at org.totschnig.drive.sync.GoogleDriveBackendProviderFactory.fromAccount(GoogleDriveBackendProviderFactory.kt:19)
at org.totschnig.drive.sync.GoogleDriveBackendProviderFactory.fromAccount(GoogleDriveBackendProviderFactory.kt:12)
at org.totschnig.myexpenses.sync.SyncBackendProviderFactory.from-0E7RQCE(SyncBackendProviderFactory.kt:18)
at org.totschnig.myexpenses.sync.SyncBackendProviderFactory.access$from-0E7RQCE(SyncBackendProviderFactory.kt:12)
at org.totschnig.myexpenses.sync.SyncBackendProviderFactory$Companion.get-0E7RQCE(SyncBackendProviderFactory.kt:41)
at org.totschnig.myexpenses.sync.SyncAdapter.onPerformSync(SyncAdapter.kt:123)
at android.content.AbstractThreadedSyncAdapter$SyncThread.run(AbstractThreadedSyncAdapter.java:354)

Caused by java.lang.InstantiationException: java.lang.Class cannot be instantiated
at java.lang.Class.newInstance(Class.java)
at com.google.api.client.util.Types.newInstance(Types.java:113)
at com.google.api.client.util.Data.createNullInstance(Data.java:166)
at com.google.api.client.util.Data.nullOf(Data.java:134)
at com.google.api.services.drive.model.File.(File.java:69)
at java.lang.Class.newInstance(Class.java)
at com.google.api.client.util.Types.newInstance(Types.java:113)
at com.google.api.client.util.Data.newMapInstance(Data.java:550)
at com.google.api.client.json.JsonParser.parseValue(JsonParser.java:766)
at com.google.api.client.json.JsonParser.parse(JsonParser.java:360)
at com.google.api.client.json.JsonParser.parse(JsonParser.java:335)
at com.google.api.client.json.JsonObjectParser.parseAndClose(JsonObjectParser.java:79)
at com.google.api.client.json.JsonObjectParser.parseAndClose(JsonObjectParser.java:73)
at com.google.api.client.http.HttpResponse.parseAs(HttpResponse.java:460)
at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.execute(AbstractGoogleClientRequest.java:603)
at org.totschnig.drive.sync.DriveServiceHelper.getFile(DriveServiceHelper.java:143)
at org.totschnig.drive.sync.GoogleDriveBackendProvider.requireBaseFolder(GoogleDriveBackendProvider.kt:403)
at org.totschnig.drive.sync.GoogleDriveBackendProvider.(GoogleDriveBackendProvider.kt:420)
at org.totschnig.drive.sync.GoogleDriveBackendProviderFactory.fromAccount(GoogleDriveBackendProviderFactory.kt:19)
at org.totschnig.drive.sync.GoogleDriveBackendProviderFactory.fromAccount(GoogleDriveBackendProviderFactory.kt:12)
at org.totschnig.myexpenses.sync.SyncBackendProviderFactory.from-0E7RQCE(SyncBackendProviderFactory.kt:18)
at org.totschnig.myexpenses.sync.SyncBackendProviderFactory.access$from-0E7RQCE(SyncBackendProviderFactory.kt:12)
at org.totschnig.myexpenses.sync.SyncBackendProviderFactory$Companion.get-0E7RQCE(SyncBackendProviderFactory.kt:41)
at org.totschnig.myexpenses.sync.SyncAdapter.onPerformSync(SyncAdapter.kt:123)
at android.content.AbstractThreadedSyncAdapter$SyncThread.run(AbstractThreadedSyncAdapter.java:354)
```