Harden /mnt/{user,installer} permission bits

These paths previously had 0755 permission bits
(/mnt/installer got its bits from the /mnt/user bind mount).
With such permissive bits, an unauthorized app can access a file using
the /mnt/installer path for instance even if access via /storage
would have been restricted.

In init.rc we create /mnt/user with 0755 initially, this is to keep
/sdcard working without FUSE. When mounting a FUSE filesystem, we
enusure in vold that /mnt/user is changed to 0700

Bug: 135341433
Test: adb shell ls -d /mnt/{user, installer}
Change-Id: Id387e34c5fd257858861246ad51486892653fb3a