log last fs shutdown time / stat before reboot

- init will record shutdown time spent in file system part
  to kernel log, and it is avaiable as last_kmsg after reboot.
- Find it from last_kmsg and log it.

bug: 36004738
Test: reboot / cold boot and check logs
Change-Id: Ic5795b327eec482895ac79b39be2a246c6997999