O.k. so i finally figured things out.
to achieve that, (getting notified about device contacts changes only), you don't need a stub content provider. simply declare a sync adapter with a sync adapter service. in the syncadapter's xml declaration, use the ContactsContract.AUTHORITY which means: android:contentAuthority="com.android.contacts"
and set an automatic sync: ContentResolver.setSyncAutomatically(account, ContactsContract.AUTHORITY, true);
remember that your onPerformSync method in your SyncAdapter implementation won't be called right away, the system "waits" for the best time to make those syncs, any way from my experience it took ~40 seconds.