[DocsUI M3] Fix the alignment for list table header

* Use start-align for Size/Date columns (previously they were
  end-aligned).
* Update typography for header texts, use Emphasized font style.
* Add a placeholder in the table header for preview icon to fix
  the alignment issue in picker mode.

Check the attached bug for demo video.

Bug: 397517567
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I94fb3e0844cc995e98c2b2fcd33196e4571d3117