libcutils: android_reboot: save reboot reason in file

Save a string identifying the reason for last Android reboot or power
off in file /data/misc/recovery/last_reboot_reason .  This file may
be used for informing users of reboot or shutdown reasons at next
boot, and for other diagnostic purposes.

Bug: 30994946
Test: Manual: reboot, setprop sys.powerctl
Change-Id: I01e44473fdd21b33e9e4dced77aba9a66b6d3755