Harden /mnt/pass_through permission bits

It previously had 0755 permission bits

With such permissive bits, an unauthorized app can access a file using
the /mnt/pass_through path for instance even if access via /storage
would have been restricted.

It is now 0700

TODO: Change ACL for /mnt/user from 0755 to 0700 in vold only when
FUSE flag is on. Changing it with FUSE off breaks accessing /sdcard
because /sdcard is eventually a symlink to /mnt/user/0/primary

Test: adb shell ls -d /mnt/pass_through
Bug: 135341433
Change-Id: I3ea9655c6b8c6b4f847b34a2d3b96784a8f4a160