Sync-related classes are not excluded from Proguard correctly

[edualonso]

Having exceptions for sync-related classes in the library-sync Proguard consumer doesn't work. JavaClassGlobalDef has references to said classes when JNI is initialised, causing it to crash.

Said classes are all located under the cinterop module, which is shared across library-base and library-sync. Given that the JNI initialiser is only present for library-base, said exceptions must also be in the library-base Proguard consumer.

Additionally, RealmObjectCompanion had not been included from being stripped, resulting in crashes too as trying to retrieve companion objects from RealmObjects would cause crashes in release builds too.