ManualControl: block race condition for arming on switch

In the case of arming on a switch, if the person turned off the
transmitter and the switch went to the arm position before the
connected flag went false, they could have an arm event that was
not desired. The disarm timeout would kick in after this.

For what it is worth when this arming happened it shouldn't have
created any issues because the throttle would be set to low for
invalid input.