[DocsUI, Search]: Fix listing of directories.

The FolderLoader was getting the same MIME type as were prepared for
search. This resulted in empty listing of directories. This CL fixes it
by computing mime types differently for search and folder listing.

Bug: 417571092
Test: atest DocumentsUIGoogleTests
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: I351e19894f8bb62afac960e48154f3c4896585c2