Hide Collection Group queries from public API until backend support is ready.

Author: Michael Lehenbauer

firebase-firestore/CHANGELOG.md

@@ -5,8 +5,6 @@
 - [fixed] Fixed calculation of SQLite database size on Android 9 Pie devices.
   Previous method could be off by a few MBs on these devices, potentially
   delaying garbage collection.
-- [feature] You can now query across all collections in your database with a
-  given collection ID using the `FirebaseFirestore.collectionGroup()` method.
 
 # 18.0.1
 - [fixed] Fixed an issue where Firestore would crash if handling write batches

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

@@ -235,6 +235,7 @@ public DocumentReference document(@NonNull String documentPath) {
     return DocumentReference.forPath(ResourcePath.fromString(documentPath), this);
   }
 
+  // TODO(b/116617988): Expose API publicly once backend support is ready (and add to CHANGELOG.md).
   /**
    * Creates and returns a new @link{Query} that includes all documents in the database that are
    * contained in a collection or subcollection with the given @code{collectionId}.
@@ -244,8 +245,8 @@ public DocumentReference document(@NonNull String documentPath) {
    * @return The created Query.
    */
   @NonNull
-  @PublicApi
-  public Query collectionGroup(@NonNull String collectionId) {
+  //@PublicApi
+  /* public */ Query collectionGroup(@NonNull String collectionId) {
     checkNotNull(collectionId, "Provided collection ID must not be null.");
     if (collectionId.contains("/")) {
       throw new IllegalArgumentException(