chore(firestore): avoid starting GC scheduler if GC is disabled (#8058)

This change avoids starting the GC scheduler if garbage collection is
disabled (i.e., when the cache size is set to unlimited).

Author: wu-hui

firebase-firestore/src/main/java/com/google/firebase/firestore/core/FirestoreClient.java

@@ -26,6 +26,7 @@
 import com.google.firebase.firestore.EventListener;
 import com.google.firebase.firestore.FirebaseFirestoreException;
 import com.google.firebase.firestore.FirebaseFirestoreException.Code;
+import com.google.firebase.firestore.FirebaseFirestoreSettings;
 import com.google.firebase.firestore.LoadBundleTask;
 import com.google.firebase.firestore.PipelineResultObserver;
 import com.google.firebase.firestore.TransactionOptions;
@@ -296,7 +297,9 @@ private void initialize(
     eventManager = provider.getEventManager();
     IndexBackfiller indexBackfiller = provider.getIndexBackfiller();
 
-    if (gcScheduler != null) {
+    if (gcScheduler != null
+        && provider.settings.getCacheSizeBytes()
+            != FirebaseFirestoreSettings.CACHE_SIZE_UNLIMITED) {
       gcScheduler.start();
     }