Shutdown

Invalid date

* Using 'reboot -p' with su command directly, so you have to grant it permissions again (and you can remove the other one) - but now you can see for sure that this app only executes the reboot command.
* Notice is given if phone is not rooted or app has no permissions to run as superuser
* Shutdown is done in background thread, so it shouldn't freeze the phone anymore
* Cleanly terminate app after 15s, if shutdown did not work
* Overall improvements, better logging