[DocsUI M3] Use Material3 Themes

* Remove the DocumentsDefaultM3Theme because now we have M3 stuff
  in the res/flag(use_material3) folder.
* Update the parent theme to the Material3 one in themes.xml from
  the res/flag(use_material3) folder.

Bug: 377771271
Test: build and run DocumentsUIGoogle with the flag on
Flag: com.android.documentsui.flags.use_material3
Change-Id: Ie87016739afeab9151d1e51179bac06863e1fce5