[DocsUI M3] Rename overlayable dimension variables

* grid_item_radius on Pixel RRO has a value which doesn't
  looks good with grid's outer radius, so remove the
  reference of this variable and use CardView's default
  radius instead.
* the same for search_chip_radius, although Pixel RRO has the
  same value as Chip's default radius, still remove the
  reference of it.

Check the attached bug for before/after comparison.

Bug: 410428036
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I84b7207cf1104320d4918e1a3ae0e5457ff8d6c0