When linking two accounts, use the provider the user last added to their account instead of the first one (#701)

Author: SUPERCILEX

auth/src/main/java/com/firebase/ui/auth/provider/ProviderUtils.java

@@ -68,7 +68,7 @@ public String then(@NonNull Task<ProviderQueryResult> task) throws Exception {
 
                         List<String> providers = task.getResult().getProviders();
                         return providers == null || providers.isEmpty()
-                                ? null : providers.get(0);
+                                ? null : providers.get(providers.size() - 1);
                     }
                 });
     }