Alter UAS.cc so that it transmits MANUAL_CONTROL messages at 5Hz when no inputs have changed.

Otherwise MANUAL_CONTROL messages are still sent at the regular 50Hz. This reduces transmission bandwidth from 0.9kbps when the joystick is used continuously to 0.1kbps when it's idle.
To prevent possible issues from the last high-bandwidth message being lost and the system then not responding for .1s, the last messsage is double-transmit which should prevent those issues.