adbd: when rebooting, don't close socket before pausing.

An accidental side-effect of the change to unique_fd was that we
started closing the reboot service socket before pausing and waiting
for our death. This results in the client immediately returning, which
means they might get a chance to run other commands before we actually
manage to reboot.

Bug: http://b/112104037
Test: adb reboot; adb devices
Change-Id: Id61321da0c08ebd1ab57f1ce926bafd9109dd9c2