Grant shell REBOOT permission

Shell already can reboot a device (via `adb reboot` command). This
change allows shell to also trigger a reboot logic inside the
system_server.

Additionally, it allows to CTS test PowerManager.reboot API.

Test: adb shell svc reboot
Change-Id: I4545c4f3587fee2840a00648c569f75bb57501fc
Merged-In: I4545c4f3587fee2840a00648c569f75bb57501fc