fix #838 prefer `updateRootMap` instead of `rootFolderPath` in NodeJS

This commit adds `DownloadMgr.updateRootMap` call in the AT loader for NodeJS, so that the user can change `Aria.rootFolderPath` to a folder of his custom files, without worrying of breaking the loading of AT internal files (`aria.*` classpath).

This is a backward-compatible commit, a prerequisite for #805 which will change the default `rootFolderPath`.

----

I've decided it's better to go the safe route here, we can merge #805 in 1.5.1.