[DocsUI M3] Uplift preview icons for grid items

Update the style and positioning of preview icons so that a
20dp preview icons appears on the top right side of the thumbnail,
a couple of pixels indented. The touch target for the button is
34x34.

See bug for demo.

Bug: 409667693
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I91c6ea9a764fa4f753433c68209365cc7b1e3cd1