Fix flight actions state

Closes https://github.com/DroidPlanner/droidplanner/issues/856

Changes:
- all buttons (exceprt Edit) in FlightActionsFragment are enabled/disabled based on isArmed() && !isFailsafe()
- as Loiter, Home and Land buttons are just shortcuts of corresponding mode, when pressed, they become disabled, and when mode is changed, enabled

TODO:
- [ ] Make drawables for disabled state of buttons in FlightActionsFragment
- [ ] Hide or disable buttons when drone not connected?