Remove Images/Videos/Audio/Documents roots on desktop

Also introduce the flag hide_roots_on_desktop to put this feature
behind. Enable this flag in ag/31625340.

Bug: 381959330
Flag: com.android.documentsui.flags.hide_roots_on_desktop
Test: atest RootsFragmentTest
Change-Id: I3c8efe177e23a20e821be97268f549fb35fe1b32