storageproxyd: Add mapping option without symlinks.

This adds an option to map files to block device without creatinga
symlink in the storage root. As a result, files can be mapped before the
storage root is mounted.

Bug: 421239916
Test: boot desktop using the new desktop specific TDP partition
Change-Id: Ida78aa2241cf5eb47f4b8cbf216b3a38b3675852