android_reboot: move reboot reason file to data/misc/reboot/

Move last reboot reason file to new directory data/misc/reboot/ to
require only SELinux permissions specific to this new file.

Bug: 30994946
Test: manual: reboot command, setprop sys.powerctl
Change-Id: I1e067235aa4b06391cff8ab0741a9d317ba5b7da
1 file changed