Minor cleanups related to composite filters (backport from iOS). (#3796)

Author: ehsannas

firebase-firestore/src/main/java/com/google/firebase/firestore/Query.java

@@ -618,7 +618,7 @@ private void validateNewFieldFilter(
         validateOrderByFieldMatchesInequality(firstOrderByField, newInequality);
       }
     }
-    Operator conflictingOp = findFilterWithOperator(query.getFilters(), conflictingOps(filterOp));
+    Operator conflictingOp = findOpInsideFilters(query.getFilters(), conflictingOps(filterOp));
     if (conflictingOp != null) {
       // We special case when it's a duplicate op to give a slightly clearer error message.
       if (conflictingOp == filterOp) {
@@ -649,7 +649,7 @@ private void validateNewFilter(com.google.firebase.firestore.core.Filter filter)
    * returns the first one that is, or null if none are.
    */
   @Nullable
-  private Operator findFilterWithOperator(
+  private Operator findOpInsideFilters(
       List<com.google.firebase.firestore.core.Filter> filters, List<Operator> operators) {
     for (com.google.firebase.firestore.core.Filter filter : filters) {
       for (FieldFilter fieldFilter : filter.getFlattenedFilters()) {

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

@@ -115,14 +115,9 @@ public CompositeFilter withAddedFilters(List<Filter> otherFilters) {
    */
   @Nullable
   private FieldFilter findFirstMatchingFilter(Function<FieldFilter, Boolean> condition) {
-    for (Filter filter : filters) {
-      if (filter instanceof FieldFilter && condition.apply(((FieldFilter) filter))) {
-        return (FieldFilter) filter;
-      } else if (filter instanceof CompositeFilter) {
-        FieldFilter found = ((CompositeFilter) filter).findFirstMatchingFilter(condition);
-        if (found != null) {
-          return found;
-        }
+    for (FieldFilter filter : getFlattenedFilters()) {
+      if (condition.apply(filter)) {
+        return filter;
       }
     }
     return null;