Fix copy/move progress message when destination is a root

A root's base document info may have a different display name to the
root's actual title. `stack.getTitle()` accounts for this, so use that
to get the display name.

Bug: 416141710
Test: atest 'DocumentsUIGoogleTests:com.android.documentsui.services'
Flag: com.android.documentsui.flags.visual_signals_ro
Change-Id: I4eba60e8e4668fc3af01a60bd5a6a74090cfa8ba