Add /data/misc/reboot and reboot_data_file context

Add a file context for keeping track of last reboot reason and label
directory /data/misc/reboot/ for this purpose.

Bug: 30994946
Test: manual: reboot ocmmand, setprop sys.powerctl, SoC thermal mgr
Change-Id: I9569420626b4029a62448b3f729ecbbeafbc3e66