Add missing function to MemoryIndexManager

Author: ehsannas

firebase-firestore/src/main/java/com/google/firebase/firestore/local/MemoryIndexManager.java

@@ -108,6 +108,11 @@ public FieldIndex.IndexOffset getLeastRecentIndexOffset(Target target) {
     return FieldIndex.IndexOffset.NONE;
   }
 
+  @Override
+  public FieldIndex.IndexOffset getLeastRecentIndexOffset(Collection<FieldIndex> fieldIndexes) {
+    return FieldIndex.IndexOffset.NONE;
+  }
+
   @Override
   public void updateIndexEntries(ImmutableSortedMap<DocumentKey, Document> documents) {
     // Field indices are not supported with memory persistence.