How can I enable Android Debugging from a root shell on the phone?
Run:
settings put global adb_enabled 1

You can also start adbd service by executing start adbd or setprop ctl.start adbd. But I prefer the settings way because it properly goes through Android framework.
PS:
For clarification, adbd listens on port 5555 only if ADB over Network is enabled (from Developer Options, or by running setprop service.adb.tcp.port 5555 on device, or by running adb tcpip 5555 on PC). In USB mode (which can be switched to by running adb usb on PC) adbd is not listening on any network port. Some more related details in this answer.