[DocsUI M3] Use use_material3 flag for hiding Image/Videos/Audio roots

Deprecate hide_roots_on_desktop_ro in place of use_material3.

Bug: 407875196
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: Id69fdd70fbfc9330939cf3c84ed6eccc4564882a