Add shutdown assurance

If something kills system_server before it completes its shutdown
action, the runtime will just restart giving the illusion that a reboot
for an OTA or something else has happened.

To prevent this, write a system property containing the reboot reason
before initiating the shutdown with all the services. If the
system_server is killed before it completes, the next time the main
thread of system_server starts up, it will immediately execute the
shutdown action.

Bug: 3022556
Change-Id: I81723bac333430f04205e7a7b799914d96f170eb