Plane: added time for flying in any mode

- previously we only kept track of starting flying while in auto which gets reset when switching in and out of auto and on takeoff/land. Now we keep track of a "global" one that will track the is_flying flag regardless of the flight mode.

ArduPlane/Attitude.cpp

@@ -577,7 +577,7 @@ bool Plane::suppress_throttle(void)
         auto_state.takeoff_complete == false) {
 
         if (is_flying() &&
-            millis() - auto_state.started_flying_in_auto_ms > 5000 && // been flying >5s
+            millis() - started_flying_ms > 5000 && // been flying >5s in any mode
             adjusted_relative_altitude_cm() > 500) { // are >5m above AGL/home
             // we're already flying, do not suppress the throttle. We can get
             // stuck in this condition if we reset a mission and cmd 1 is takeoff

ArduPlane/Plane.h

@@ -512,6 +512,9 @@ private:
     // previous value of is_flying()
     bool previous_is_flying;
 
+    // time since started flying in any mode in milliseconds
+    uint32_t started_flying_ms;
+
     // Navigation control variables
     // The instantaneous desired bank angle.  Hundredths of a degree
     int32_t nav_roll_cd;

ArduPlane/is_flying.cpp

@@ -99,6 +99,11 @@ void Plane::update_is_flying_5Hz(void)
 
         auto_state.last_flying_ms = now_ms;
 
+        if (!previous_is_flying) {
+            // just started flying in any mode
+            started_flying_ms = now_ms;
+        }
+
         if ((control_mode == AUTO) &&
             ((auto_state.started_flying_in_auto_ms == 0) || !previous_is_flying) ) {