Fix flight actions state

Closes https://github.com/DroidPlanner/droidplanner/issues/856

Changes:
- All buttons (except Edit) in FlightActionsFragment are shown/hidden based on isArmed() && !isFailsafe()
- As Loiter, Home and Land buttons are just shortcuts of corresponding modes, when clicked, they become disabled, and when mode is changed, enabled

TODO:
- [ ] Prepare drawables for disabled state of buttons in FlightActionsFragment
- [x] Hide or disable buttons? A: Look Changes