[DocsUI M3] Fix thumbnail corner pixelation

* Use ShapeableImageView to achieve thumbnail with round corner and
  border.
* Revert the thumbnail loaded callback added in CL:31732344 because
  now we have border directly on the thumbnail image view, not its
  parent wrapper.

Check the attached bug for demo.

Bug: 403413105
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I6ae475a1472c595c88d9090d18837b7a49f9a0fc