[client-sync] Fix runtime error introduced in 08975c0

Summary:
If `gmailInitialUIDsRemaining` was defined, we need to set `initialUids`
to that value. We were previously setting it to a key in
`folder.syncState` that didn't exist, which caused a runtime error.

This was introduced in d6a2b6935c

Test Plan: manual

Reviewers: spang, mark

Reviewed By: mark

Differential Revision: https://phab.nylas.com/D4279

packages/client-sync/src/local-sync-worker/sync-tasks/fetch-messages-in-folder.imap.es6

@@ -405,7 +405,7 @@ class FetchMessagesInFolderIMAP extends SyncTask {
           fetchedmax: this._box.uidnext,
         });
       } else {
-        initialUids = this._folder.syncState.gmailInitialUIDsRemaining;
+        initialUids = gmailInitialUIDsRemaining
       }
       // continue fetching new mail first in the case that inbox uid download
       // takes multiple batches