[DocsUI M3] Uplift hover and focused state for grid item thumbnail

Implement the selected+focused+hovered and focused+hovered states for
grid item thumbnails. These states were only handled for the nameplate
but now they are handled for the thumbnail too.

Also update the thumbnail radius to 20 dp to match the figma specs.

See bug for screenshots.

Bug: 401992851
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: Ib940db134363fa8a2838820b6f18ab7dee56490c