Switch /data/misc/reboot/last_reboot_reason to persistent property

Switch from /data/misc/reboot/last_reboot_reason to persistent
Android property persist.sys.boot.reason for indicating why the
device is rebooted or shutdown.

persist.sys.boot.reason has a standard as outlined in b/63736262 and
the associated investigation. Made adjustments to the values so that
we did not create a problem even before we started. Compliance is
part of the tests in boot_reason_test.sh.

Test: system/core/bootstat/boot_reason_test.sh
Bug: 64687998
Change-Id: Iba69acf2105f4446411d86cdb8097a1755a20f15