TabletControl: set flight mode to PathPlanner when used

This is a temporary commit. It produces the right behavior, because
the tablet control code setting  new PathDesired values is the same
as the behavior when running the path planner. However, it needs to
be written as a different mode to allow the PathPlanner to be used
as well.
TabletControl: use TabletControl as flight mode

This indicates that the tablet_control.c code is setting
PathDesired directly, similarly to when using PathPlanner.
TabletControl: increase the altitude to 15m

This gives a lot more clearance. This should eventually be
programmable or on the tablet display.