MAVLink: added RAW_SENSORS and RC_CHANNELS to preferences

Added RAW_SENSORS and RC_CHANNELS to preferences (Settings -> MavLink -> Stream Rates) with default values 2 and 5 respectively.

RAW_SENSORS and RC_CHANNELS MAVLink data stream rates were not exposed in settings UI, code set both to 0.

This caused some MavLink devices requiring these datas-streams to stop working after connecting to the vehicle with DroidPlanner, eg jDrones IO board running MavLink based LED strip controller.

We were able to reproduce this on a multi-rotor camera ship this afternoon - the LED controller would become unresponsive after connecting with DroidPlanner. The LED controller board was relying on the MavLink RC_CHANNELS data-stream to monitor channel inputs - DP was reconfiguring this data stream to baud rate 0 (disabled).

This fix solves this problem (thanks for finding this one Mike W).