Give system_server access to last reboot reason

system_server needs to be able to read the file
last_reboot_reason to see if we need to show
a thermal notification to the user. This change
allows system_server to read, delete, and rename
the file.

Test: Manual
Bug: 30994946
Change-Id: Id73e18768f910c8de0bfc02f1b464e689f4b3955