Mount fstab.postinstall

Mount fstab.postinstall

Instead of using the 'mount' built-in, do a
mount_all /system/etc/fstab.postinstall which mounts
system_other to postinstall properly. This works for
devices with and without dynamic partitions.

- retrofit and non DAP devices mount the physical system_other partition
- launch devices mount the logical system_other partition

Fixes: 113182233
Test: boot the first time and look at cppreopt results
      (on retrofit and launch DAP devices)

Change-Id: I70aef55235a5b8be34a521774c4e9dc8461b84de