Follow Me Mode Change Failsafe - v11.0.0

Add follow me mode change failsafe, and the below too.

* Give errors if drone is not connected or armed if trying to turn on follow me
* Turn off follow me if GPS service on the Android device is turned off
* Remove magic numbers for USB connection timeouts and change to constants
* Send correct sequence number for transmitted MAVLink packets to the vehicle
* Set guided mode when Follow Me is enabled
* Set loiter mode when Follow Me is disabled
* Declare and register a second mode change listener for the follow me mode failsafe to function