Notify: APM2 arming buzzer to long tone

Arming tone is a solid 3 second tone so it is consistent with Pixhawk
arming tone
Copter: fix arming tone to sound before arming

Also fixed bug in which vehicle could appear to be armed (according to
LEDs) but is not.  This could occur if pilot had raised the throttle
very high during the calibration that happens just before arming.