[DocsUI M3] Show the thumbnail stroke when selected too

Show the stroke around the thumbnail always, including
when the item is selected. This follows the figma spec.

Bug: 381962394
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I6f72fcf4e1b04375e7a3413ae71e613ae410b23d