[DocsUI M3] Fix the corner pixelation for thumbnail

Thumbnail corner in list/grid view shows a weird pixelation
issue, this CL applies a custom outline to the ImageView
which holds the thumbnail to fix the issue.

Check the attached bug for the before/after comparison.

Bug: 403413105
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I8ac34e5b0fc7d92d36006a9e2bb8315ef8cb5dca