return unmodifiable map

Author: tejasd

firebase-crashlytics/src/main/java/com/google/firebase/crashlytics/internal/metadata/UserMetadata.java

@@ -22,6 +22,8 @@
 import com.google.firebase.crashlytics.internal.concurrency.CrashlyticsWorkers;
 import com.google.firebase.crashlytics.internal.model.CrashlyticsReport;
 import com.google.firebase.crashlytics.internal.persistence.FileStore;
+
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -163,7 +165,7 @@ public Map<String, String> getCustomKeys(Map<String, String> eventKeys) {
                   + MAX_ATTRIBUTES);
       break;
     }
-    return result;
+    return Collections.unmodifiableMap(result);
   }
 
   /**