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: I812c55a12faf7cb7ff92101009be058ad9958d07
diff --git a/init/reboot.h b/init/reboot.h
index 8586556..ece407f 100644
--- a/init/reboot.h
+++ b/init/reboot.h
@@ -27,7 +27,7 @@
 
 /* Reboot / shutdown the system.
  * cmd ANDROID_RB_* as defined in android_reboot.h
- * reason Reason string like "reboot", "userrequested"
+ * reason Reason string like "reboot", "shutdown,userrequested"
  * rebootTarget Reboot target string like "bootloader". Otherwise, it should be an
  *              empty string.
  * runFsck Whether to run fsck after umount is done.