Move display mode switch icon to sub header

Based on designer mock dock, grid/list mode switch icon should
place on sub header rather than tool bar.

Remove apps row item min height to avoid too many white space
when all items in apps row is provider.

Remove some unused import.

Fix: 123569812
Test: manual
Test: atest DocumentsUITests
Change-Id: I4777b638764dd4a52be6b65e7d8adbd1f60beffa