[DocsUI M3] Uplift folders in grid

Use the new grid item layout for folders. This means that the folder
and file item logic will be combined into the GridDocumentHolder.
The following differences between GridDirectoryHolder and
GridDocumentHolder have been folded into GridDocumentHolder:
- inSelectRegion() is only activated for ACTION_BROWSE for folders.
   - This condition is now checked for folders.
- There is no previewIcon (bindPreviewIcon()) for folders.
   - bindPreviewIcon() now exits early for folders
- There is no image thumbnail for folders, the folder mime icon is
hardcoded in instead.
   - mIconHelper.load() now handles finding the folder mime icon, which
     is the same logic used by ListDocumentHolder which also handled
     both files and folders.

See b/400807990 #comment2 for before and after screenshots.

Bug: 400807990
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I0ee8725b8f93f4b53e9f2e02cb95b8ba93406786