plane: Report MAV_STATE_ACTIVE if flying, MAV_STATE_STANDBY otherwise

(There are also various error states MAV_STATE_CRITICAL that are unchanged
by this check in).

The is_flying() check is similar to the code in suppress_throttle, but
I wanted to make it very optimistic on thinking we are flying - which is
not what you want for set_throttle...

(AC has similar MAV_STATE_ACTIVE vs MAV_STATE_STANDBY behavior now)