Revert^2 "[DocsUI M3] Rename PickFragment to PickDirectoryFragment"

This reverts commit 1f27ac89.

Reason for revert: The original revert was caused by renaming
fragment_pick.xml to fragment_pick_directory.xml only behind the
use_material3 flag. This can be fixed by renaming it when the flag is
off too.

Bug: 409931248
Bug: 409985958
Change-Id: I2e217605bfa67d6bd2cfa263f9c0301d370a266e