Minimal support for sending device rotation through UDP towards OpenTrack.

This makes LimeLight usable for 'looking around' in supported games while
using Cardboard.

OpenTrack is intended to be running on the same desktop. Port 4242 is
hardcoded. Headtracking data is always sent -- it can't be turned off.
Only rotation is sent; X, Y and Z are hardcoded to be zero.

Due to use of Sensor.TYPE_GAME_ROTATION_VECTOR, minimal API version has
been bumped to 18.

As this is only for my personal use, all these caveats are acceptable.