emoji [nfc]: Read _popularCandidates only through public getter

Later in this series, we'll make _popularCandidates nullable, as a
caching implementation detail behind popularEmojiCandidates. (Both
will become non-static.)

Author: chrisbobbe

lib/model/emoji.dart

@@ -320,7 +320,7 @@ class EmojiStoreImpl extends PerAccountStoreBase with EmojiStore {
 
     // Include the "popular" emoji, in their canonical order
     // relative to each other.
-    results.addAll(_popularCandidates);
+    results.addAll(EmojiStore.popularEmojiCandidates);
 
     final namesOverridden = {
       for (final emoji in activeRealmEmoji) emoji.name,