DocsUI M3: add show_media_roots config

Showing (or hiding) these roots is now configured by an RRO instead of
via FEATURE_PC.

Bug: 407875196
Bug: 414695617
Test: atest DocumentsUIGoogleTests:com.android.documentsui.FilesActivityDefaultsUiTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: Ic3c6085fcf3fac0afae6de9ad2666c6ccae4704d