EKF: Rework nav validity reporting

Remove duplicate checking for dead reckoning and consolidate into a single function.
Use separate timers to check for start of dead reckoning and check when dead reckoning has been performed for too long for the nav solution to be valid.
Allow the timeout for validity reporting to be adjusted externally.
Separate external reporting of dead reckoning from internal checks.

EKF/ekf.h

@@ -173,8 +173,8 @@ public:
 	// return true if the global position estimate is valid
 	bool global_position_is_valid();
 
-	// return true if the EKF is dead reckoning the position using inertial data only
-	bool inertial_dead_reckoning();
+	// check if the EKF is dead reckoning horizontal velocity using inertial data only
+	void update_deadreckoning_status();
 
 	// return true if the terrain estimate is valid
 	bool get_terrain_valid();
@@ -291,6 +291,7 @@ private:
 	bool _tas_data_ready{false};	///< true when new true airspeed data has fallen behind the fusion time horizon and is available to be fused
 
 	uint64_t _time_last_fake_gps{0};	///< last time we faked GPS position measurements to constrain tilt errors during operation without external aiding (uSec)
+	uint64_t _time_ins_deadreckon_start{0};	///< amount of time we have been doing inertial only deadreckoning (uSec)
 
 	uint64_t _time_last_pos_fuse{0};	///< time the last fusion of horizontal position measurements was performed (uSec)
 	uint64_t _time_last_delpos_fuse{0};	///< time the last fusion of incremental horizontal position measurements was performed (uSec)