[DocsUI M3] Split the main container into 3 sections

* Split the main container section (right part) into
  3 sub sections: app bar and search chips, file list/grid,
  file path bar (breadcrumb), add gap between the 3
  sub sections and add corner radius. Note: this doesn't
  impact the compact layout.
* Remove the additional bottom padding for both nav
  tree and directory list in full screen mode.
* Removes the bottom border for table header as per spec.
* Explicitly specify background color for breadcrumb in
  compact screen to avoid overlap between the file list
  and the breadcrumb area.

Check the attached bug for demo video.

Bug: 387869597
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: Ib7f7e8837d7486a0e902604386f802322e557ab0