Harden /mnt/pass_through paths

Even though /mnt/pass_through itself is 700 root root, the paths under
it are quite permissive. Now, change them from 755 to 710 root
media_rw since the FUSE daemon is the only one that should access it
and it has media_rw gid

Test: manual
Bug: 135341433
Change-Id: I743c014f2c0273c68a1cead7f4331b55a3abcb4e