Handle the reboot in the recovery user mode

We have switched to "/system/bin/reboot" to perform the reboot. But
accessing the shell for user build under recovery is not allowed by
sepolicy. So adding a special routine to handle this reboot case.

Bug: 141247819
Test: run `adb reboot` in user mode recovery
Change-Id: Ia37438e52c67fd2bd54e923a0fea5817a1455a08