[DocsUI] Add item_root to medium non Material 3 directory list layout

The existing non Material 3 medium screen size layout does not put the
item_root ID on the file row, which the other layouts do. This was
causing tests in PickerPreviewAllTypeUiTest to fail.

Flag: EXEMPT bugfix
Bug: 430829388
Test: atest DocumentsUIGoogleTests.com.android.documentsui.PickerPreviewAllTypeUiTest
Change-Id: I5859a27233c6a4d011b388ffdf3bf9f6ea826d26