Sets default value of ro.postinstall.fstab.prefix

ro.postinstall.fstab.prefix might be either "/system" or "/product",
to decide the location of the fstab.postinstall, used to mount
system_other partition on A/B devices.

  {ro.postinstall.fstab.prefix}/etc/fstab.postinstall

Bug: 112103720
Test: factory reset and boot device, checks cppreopt logs

Change-Id: Ib1e282752c37713e2220239f4f903453ce3c8bab