Update the legacy sensor code to use SensorEventListener instead of t…

…he (#451)

* Update the legacy sensor code to use SensorEventListener instead of the
deprecated SensorListener. This also allows for some sanitization of the
code since we no longer have to deal with various coordinate systems
being backwards.

* Update the docs some more.

designdocs/sensors.md

@@ -93,14 +93,14 @@ Since these vectors are all normalised we can invert them by taking the transpos
 `M = [N_c | U_c | E_c] * [N_p | U_p | E_p]_T` 
 
 So the only thing remaining is to figure out `P_p` and `Q_p` which are usually going to be
+
 [0, 0, -1] and [0, 1, 0] respectively.
 
 'Usually' because in certain circumstances you might want them to be different. For example if you
 set `P_p` to be [0, 1, 0] and `Q_p` to be [0, 0, 1] you now will have the screen showing what the
 long edge of the phone is pointing at. This can be useful if using Sky Map with a telescope - 
 you can strap it to the telescope tube.
 
-
 ## Magnetic correction
 
 All of the above assumes that the phones sensors give you true North, which they don't.