Bug 1682196 - Only allow dragging account tree children for reordering. r=mkmelin

Differential Revision: https://phabricator.services.mozilla.com/D110541

--HG--
extra : amend_source : ea689fb12c767a4420867867e56945ba7688d5a3

mailnews/base/prefs/content/AccountManager.js

@@ -1836,7 +1836,7 @@ var gAccountTree = {
       }
     });
 
-    accountTree.addEventListener("dragstart", event => {
+    mainTree.addEventListener("dragstart", event => {
       if (getCurrentAccount()) {
         event.dataTransfer.effectAllowed = "move";
         event.dataTransfer.dropEffect = "move";