[FilesTS] Add getTreeItemEntry to conditionally retrieve entry property

The `DirectoryItem` currently stores an entry on the actual UI widget.
With the new `XfTreeItem` that logic needs to be decoupled from the UI.
To avoid this, introduce a new function called `getTreeItemEntry` which
extracts the entry in different ways depending on if the new tree is
enabled or not.

Fixed: b/313011117
Change-Id: I9669d0cc1a8585e28994655d9557416a97af896e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5086974
Reviewed-by: Wenbo Jie <wenbojie@chromium.org>
Commit-Queue: Ben Reich <benreich@chromium.org>
Commit-Queue: Wenbo Jie <wenbojie@chromium.org>
Auto-Submit: Ben Reich <benreich@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1233741}