adb: Don't synchronize with transport using sleep

Services that cause adbd to reboot are currently implemented this way:
    write(fd, response)
    close(fd)
    sleep(1)
    exit(1)
sleep(1) is necessary to leave time for the transport to transmit
the response before the daemon exits. This is slow and unreliable.
Instead, add a flag to the asocket to make it exit the daemon only
after the socket is closed.

Change-Id: I9df45ea6221f0d9b828703b9b2316a5d4fe59352