Make count queries publicly available for use

Author: dconeybe

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

@@ -28,7 +28,7 @@
  * in test mocks. Subclassing is not supported in production code and new SDK releases may break
  * code that does so.
  */
-class AggregateQuery {
+public class AggregateQuery {
   // The base query.
   private final Query query;
 

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

@@ -27,7 +27,7 @@
  * in test mocks. Subclassing is not supported in production code and new SDK releases may break
  * code that does so.
  */
-class AggregateQuerySnapshot {
+public class AggregateQuerySnapshot {
 
   private final long count;
   private final AggregateQuery query;

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

@@ -15,7 +15,7 @@
 package com.google.firebase.firestore;
 
 /** Configures the behavior of {@link AggregateQuery#get}. */
-enum AggregateSource {
+public enum AggregateSource {
   /**
    * Reach to the Firestore backend and surface the result verbatim, that is no local documents or
    * mutations in the SDK cache will be included in the surfaced result.

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

@@ -1230,7 +1230,7 @@ private void validateHasExplicitOrderByForLimitToLast() {
    *     the result set of this query.
    */
   @NonNull
-  AggregateQuery count() {
+  public AggregateQuery count() {
     return new AggregateQuery(this);
   }